If we compare the results of laboratory analyses of the nutrient content in dry matter from the previous two experiments (experiments #6 and #7), we get the following picture, which can give us some idea of the potential causes of poor growth in the tested plants and suggest the direction we should take in our search for optimal recipes:

Personally, I draw the following conclusions from the above:

Cations

• Potassium (K⁺)
  • Observed pattern: Higher K input → higher tissue K (strong correlation)
  • Internal concentration (in dry matter):
    • Deficiency: <1.0% K in dry matter
    • Sufficiency: 1-3% K
  • External concentration (in water column):
    • Deficiency (<1.0%): ~1 mg/ℓ in tolerant species (probably less in sensitive species)
    • Excess/Toxicity (>3%): above ~10 mg/ℓ in tolerant species (probably much less in sensitive species)
      • An external concentration of 15 mg/ℓ K resulted in a dramatically increased potassium content in dry matter → above 6% K (concentration of 20+ mg/ℓ K can even resulted in toxic content of potassium in dry matter). This is expected as potassium is highly mobile and readily taken up when available.
    • Sufficiency (1-3%): ~2 mg/ℓ
      • Since a concentration of only 2 mg/ℓ resulted in a perfectly adequate amount of potassium in plant tissue (~3% K), higher concentrations are probably unnecessary.
      • 1-2 mg/ℓ K appears to be optimal
• Calcium (Ca²⁺)
  • Observed pattern: Higher Ca input → higher tissue Ca (weaker correlation)
  • Internal concentration (in dry matter):
    • Deficiency: <0.5% Ca in dry matter
    • Sufficiency: 0.5-2.0% Ca
  • External concentration (in water column):
    • Excess/Toxicity (>2%): <unknown>
      • Overdosing on calcium seems unlikely.
    • Sufficiency (0.5-2%): ~3 mg/ℓ
      • Since a concentration of only 3 mg/ℓ resulted in a perfectly adequate amount of calcium in plant tissue (~2% Ca), higher concentrations are probably unnecessary.
      • 2-3 mg/ℓ Ca appears to be optimal
• Magnesium (Mg²⁺)
  • Observed pattern: none (there seems to be no direct correlation between higher Mg input and its tissue concentration)
  • Internal concentration (in dry matter):
    • Deficiency: <0.1% Mg in dry matter
    • Sufficiency: 0.1-0.5% Mg
  • External concentration (in water column):
    • Excess/Toxicity (>1%): <unknown>
      • Overdosing on magnesium seems unlikely.
    • Sufficiency (0.1-0.5%): ~1 mg/ℓ
      • Since a concentration of only 1 mg/ℓ resulted in a perfectly adequate amount of magnesium in plant tissue (~0.5% Mg), higher concentrations are probably unnecessary.
      • 1 mg/ℓ Mg appears to be enough for luxurious uptake

Anions

• Nitrate (NO₃⁻)
  • Observed pattern: Higher N input → higher tissue N (weaker correlation)
  • Internal concentration (in dry matter):
    • Deficiency: <2% NO₃ in dry matter
    • Sufficiency: 2-4% NO₃
  • External concentration (in water column):
    • Deficiency (<2%): <2 mg/ℓ in tolerant species (less in sensitive species)
    • Excess/Toxicity (>4%): above ~20-30 mg/ℓ
      • Toxicity of nitrate seems unlikely, but overdose can occur at external nitrate concentrations of around 20-30 mg/ℓ which result in N concentrations in dry matter of over 5% even in some tolerant species.
    • Sufficiency (2-4%): ~3 mg/ℓ (a little more in some tolerant species)
      • Since a concentration of only 3 mg/ℓ resulted in a perfectly adequate amount of nitrate in plant tissue (~3-3.5% NO₃), higher concentrations are probably unnecessary.
      • 3 mg/ℓ NO₃ appears to be optimal
• Phosphate (H₂PO₃⁻)
  • Observed pattern: none (there seems to be no direct correlation between higher P input and its tissue concentration)
  • Internal concentration (in dry matter):
    • Deficiency: <0.2% H₂PO₃ in dry matter
    • Sufficiency: 0.2-0.7% H₂PO₃
  • External concentration (in water column):
    • Deficiency (<0.2%): <0.2 mg/ℓ in tolerant species (probably less in sensitive species)
    • Excess/Toxicity (>0.7%): <unknown>
      • Overdosing on phosphate seems unlikely.
    • Sufficiency (0.2-0.7%): ~0.3 mg/ℓ
      • Since a concentration of only 0.3 mg/ℓ resulted in a perfectly adequate amount of nitrate in plant tissue (0.3-0.5% H₂PO₃), higher concentrations are probably unnecessary.
      • 0.3 mg/ℓ H₂PO₃ appears to be optimal

Microelements

• Iron (Fe)
  • Observed pattern: ecotype specific
  • Internal concentration (in dry matter):
    • Deficiency: <50-75 ppm Fe in dry matter
    • Sufficiency: 75-400 ppm Fe
  • External concentration (in water column):
    • Deficiency (<75 ppm): ~20 µg/ℓ
      • Since a concentration of 20 µg/ℓ resulted in a deficient amount of iron in plant tissue (40-70 ppm Fe) in several species, higher concentration may be needed in most cases.
    • Excess/Toxicity (>400 ppm): >100 µg/ℓ in sensitive species (much more in tolerant species)
    • Sufficiency (75-400 ppm): ~40-60 µg/ℓ
      • Since a concentration of 20 µg/ℓ resulted in a deficiency and concentrations of 60 µg/ℓ resulted in an increased amount of iron in plant tissue (>300 ppm in some sensitive species), something in between (like 40 µg/ℓ) may be optimal.
      • 40 µg/ℓ Fe may be optimal for sensitive species (but further testing is needed)

In this experiment I used the following greenhouse (i.e. emersed-grown) plants:

1. Alternanthera reineckii 'Rosanervig'
   • (many appear to be the common A. reineckii rather than the 'Rosanervig' variety)
2. Ammannia pedicellata 'Gold'
3. Hygrophila corymbosa
4. Rotala wallichii

Lights

Filtration

Temperature

Substrate

Carbon dioxide (CO₂)

• If no specific concentration of a nutrient is listed in columns #2 to #4, it means that the concentration of that nutrient is the same as in tank #1.
• If no specific concentration of a nutrient is listed in columns #6 to #8, it means that the concentration of that nutrient is the same as in tank #5.
• The same recipe as in tank #3 was used in tanks 5-8, except for the microelements.
• The gray values in the upper right corner indicate the concentration in µEq/ℓ.
• The value in the orange triangle indicates the total nitrogen concentration converted to NO₃⁻ (e.g., 1.4 mg/ℓ NH₄⁺ and 10 mg/ℓ NO₃⁻ together contain the same amount of nitrogen as 15 mg/ℓ NO₃⁻).

Renewal frequency

• Half of the water is changed every week (usually on Saturdays). The above values represent the nutrient concentrations in the newly replenished water (except CO₂, which is a continuously [24/7] maintained concentration).
• All nutrients are added to the water once a week (macronutrients are dosed at water change, micronutrients a day or two later).

Setting the target pH using bicarbonates and/or CO₂

• Many popular aquarium plants seem to prefer soft, slightly acidic water (with a pH of around 5.5). However, to achieve such acidic water, we basically have only two options: (1) if we do not want to add any CO₂ to the water, then we must completely omit bicarbonates (i.e., KH=0), or (2) maintain bicarbonates at a very low level (KH=<1) so that they do not raise the pH too much, and then reduce this slightly elevated pH to the desired level using CO₂. Since even a small increase in CO₂ leads to a relatively dramatic improvement in growth and overall condition in most plants, I use the second method in my experiments. This has one more significant advantage (compared to the first method) → during these experiments, there is an inevitable steady decrease in pH due to the activity of nitrifying bacteria, and an appropriate addition of bicarbonates (HCO₃⁻) helps to maintain the pH at the desired level.

Initial state

Notes

• Day zero: 2025-10-09 (YYYY-MM-DD)
• Day #2: microelements dosed

Overall view  ^{Day #2}

#1-4

#5-8

Week #1  ^{Calendar week #42 (2025)}

Notes

• 2025-10-18: photos taken after water change
  • Rotala wallichii moved to the back so that it does not obstruct the view of other plants.
  • Due to the excessive drop in pH, I added about 40 µmol/ℓ (0.1°dKH) of bicarbonates to the recipe and made a 100% water change to bring the pH back to the desired level, but unfortunately, it didn't help much.
• 2025-10-19: microelements dosed

Overall view  ^{Day #9}

Show annotation

4-3-1 mg/ℓ K:Ca:Mg
 15-2 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

4-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

2-6-2 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   10 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   20 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   80 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
  160 µg/ℓ Fe

Week #2  ^{Calendar week #43}

Notes

• 2025-10-24: photos taken before water change
  • Since Rotal wallichii had grown to the surface in practically all aquariums, I transplanted it everywhere [at the water change], leaving about 20 pieces of 10 cm long growth tips in each aquarium.
    • All stems in all aquariums looked healthy and without deformities, with only slight differences in coloration (in aquarium #5, the tips were pinkish, while in most others, their color was darker, more brownish).
  • Althernathera suffered from significant leaf curling in almost all aquariums. However, in aquariums #5-8, the deformations appeared to be milder.
• 2025-10-26: water change
  • I started adding about 160 µmol/ℓ (0.45°dKH) of bicarbonates to all aquariums to raise the pH to the desired level (pH ≈ 5.5), which finally yielded positive results.

Overall view  ^{Day #15}

Show annotation

4-3-1 mg/ℓ K:Ca:Mg
 15-2 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

4-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

2-6-2 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   10 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   20 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   80 µg/ℓ Fe

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
  160 µg/ℓ Fe

Week #3  ^{Calendar week #44}

Notes

• 2025-10-27: microelements dosed
  • I am not sure if it was caused by the application of a concentrated CaCO₃ solution or the application of a concentrated microelement solution, but a number of growth tips on H. corymbosa appeared to be "burned" (scorched).
  • The upper leaves of most stems of A. reineckii and A. pedicellata began to become quite significantly twisted or stunted.
• 2025-11-01: photos taken after water change
  • Plants and aquarium walls mechanically cleaned of algae.
  • This time, bicarbonates and microelements were added to the new (replaced) water before it was poured into the aquariums to prevent direct contact between the growing tips of the plants and the concentrated solution of bicarbonates/microelements.
  • Rotala seems to grow almost identically everywhire, except that in aquariums #5 (with the lowest concentrations of microelements) it has pinkish upper leaves.
  • Hygrophila seems to grow almost identically everywhere (except for some deformities, which were probably caused by improper fertilizer application), except that in aquariums #5 and #6 (with the lowest concentrations of microelements) it has pinkish upper leaves.
  • Alternanthera grows quite well everywhere, but in most cases it has significantly twisted leaves.
  • Ammannia seems to be in the best condition in aquariums #2, #4, #7 and especially #8.
    • Tank #1: Stunted tops, some lower leaves starting to turn brown; except for a few of the newest leaves, the upper leaves are not orange-pink in color (as in most other aquariums) → worst condition, most algae.
    • Tank #2+4: The condition of the plants in aquariums 2 and 4 is very similar → one of the best condition.
    • Tank #3: Subjectively, the upper leaves are slightly more wavy than in aquariums 2 and 4.
    • Tank #6: I see almost no difference between aquariums 3 and 6 (although aquarium 6 may have slightly more wavy leaves).
    • Tank #5: Compared to aquarium 6, it may have slightly more deformed growth tips.
    • Tank #7+8: Subjectively, the least deformed leaves (aquarium 8 appears to be in the best condition overall); in both aquariums, the upper leaves are less orange-pink and more yellow (pink is almost completely absent).

Overall view  ^{Day #23}

Show annotation

#1

pH 5.8

4-3-1 mg/ℓ K:Ca:Mg
 15-2 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   39 µS/cm

#2

pH 5.6

4-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   34 µS/cm

#3

pH 5.7

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   23 µS/cm

#4

pH 5.7

2-6-2 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   61 µS/cm

#5

pH 5.7

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   10 µg/ℓ Fe

   18 µS/cm

#6

pH 5.9

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   20 µg/ℓ Fe

   22 µS/cm

#7

pH 5.0

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   80 µg/ℓ Fe

   31 µS/cm

#8

pH 4.9

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
  160 µg/ℓ Fe

   32 µS/cm

Details  ^{Day #23}

Rotala wallichii

Rotala by day

Rotala by night

Week #4  ^{Calendar week #45}

Notes

• 2025-11-10:
  • tank 1: most algae, most biofilm on water surface
  • tank 8: tank with the second highest occurrence of algae, but significantly less than in 1
  • tank 7: tank with the third highest occurrence of algae
  • tanks 2,3,4,5,6: cleanest water
  • tank 6: cleanest water surface

Overall view  ^{Day #32}

Show annotation

#1

pH 5.3

4-3-1 mg/ℓ K:Ca:Mg
 15-2 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   12 µS/cm

#2

pH 4.7

4-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   20 µS/cm

#3

pH 4.9

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   13 µS/cm

#4

pH 5.0

2-6-2 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   40 µg/ℓ Fe

   47 µS/cm

#5

pH 5.8

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   10 µg/ℓ Fe

   10 µS/cm

#6

pH 5.5

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   20 µg/ℓ Fe

   9 µS/cm

#7

pH 4.8

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
   80 µg/ℓ Fe

   20 µS/cm

#8

pH 4.7

2-3-1 mg/ℓ K:Ca:Mg
5-0.7 mg/ℓ NO₃:PO₄
  160 µg/ℓ Fe

   20 µS/cm

Subjective assessment

Ammannia pedicellata 'Gold'

#1

#2

#3

#4

#5

#6

#7

#8

• Ammannia pedicellata 'Gold'
  • 1: stunted, stagnant, practically no growth
  • 2,3,4,6: practically the same (good) condition, only slightly wavy leaves + more pronounced pink-orange colored tips (which probably indicates a slight iron deficiency)
  • 5: same as [2,3,4,6], only the tips are more pink (probably the greatest iron deficiency)
  • 7,8: almost the same, tips only slightly orange, without a pink tint (probably sufficient level of iron) → probably the best condition

Alternanthera reineckii 'Rosanervig'

#1

#2

#3

#4

#5

#6

#7

#8

• Alternanthera reineckii 'Rosanervig'
  • 1: probably the most deformed and at the same time most affected by algae
  • 2,3: same size, but in tank #3 stunted growth tips; in tank #2 in fairly good condition, only slight deformities
  • 4: some plants the same size as in [2,3], some larger, but all with stunted growth tips
  • 5: similarly deformed leaves as in [1,2,3,4], but growth tips not stunted
  • 6: one plant significantly larger than the others (the largest and most robust of all → probably outlier), but all have wavy leaves
  • 7,8: approximately the same condition, but in tank #8 possibly slightly larger biomass

Hygrophila corymbosa

#1

#2

#3

#4

#5

#6

#7

#8

• Hygrophila corymbosa
  • 1: best condition, largest and greenest
  • 2,3: same size, second best condition, but upper leaves more reddish [than in 1]
  • 4: same condition as in [2,3], but slightly smaller
  • 5,6: roughly the same size as in 4, but upper leaves pinkish; in 5, the pink is more pronounced (probably a sign of iron deficiency)
  • 7,8: same size, but overall the smallest, with severely burnt (necrotic) upper leaves (probably due to an inappropriate method of applying the microelement solution)

Rotala wallichii

#1

#2

#3

#4

#5

#6

#7

#8

• Rotala wallichii
  • 1: same size as in [3,4,5], but probably the best colored (rich red tips)
  • 5: pinkest tips (probably severe iron deficiency)
  • 3,4,6: same height, with yellow-green tips
  • 2: same condition as in [3,4,6], but slightly larger and perhaps slightly more robust (wider)
  • 7,8: largest, better colored than in [2,3,4,6], but again slightly worse colored than in 1

Objective data

Legend:

%

ppm

State

C

N

P

K

Ca

Mg

S

Na

Cl

Fe

Mn

B

Zn

Cu

Mo

Deficiency

less than normal

Sufficiency

35-45

2-4

0.2-0.7

1-3

0.5-2.0

0.1-0.5

0.15-0.5

?

0.05-0.3

75-400

20-300

10-50

20-100

2-20

0.2-10

Excess

slightly more than normal

Toxicity

significantly more than normal

Notes:

• The ranges of deficiency, sufficiency (normal), and excess (toxicity) were taken from data applicable to terrestrial plants and adapted for aquatic plants using artificial intelligence (taking into account their physiological differences). However, I would like to point out that there is not any definitive standard (norm) for freshwater aquatic plants, so all I can offer is but a qualified estimate. I leave it up to the reader to evaluate and interpret this data in their own way.
• Where I had sufficient new material available, I used only this new material for analysis. In exceptional cases (e.g., Ammannia), I also used some of the old material (i.e., original leaves/stems). However, I never used roots.

Hygrophila corymbosa

	% mg/ℓ							ppm µg/ℓ
Tank	C CO₂	N NO₃	P H₂PO₄	K	Ca	Mg	Na	Fe DTPA	Mn EDTA	Zn EDTA	Cu EDTA
8.1 pH 5.3	40.43  10	3.86  15	0.37  2	4.42  4	3.29  3	0.68  1	0	56  40	83  10	63  2	4.2  1
8.2 pH 4.7	40.83  10	2.15  5	0.17  0.7	3.88  4	2.77  3	0.56  1	0	36  40	65  10	50  2	8.1  1
8.6 pH 5.5	41.77  10	2.51  5	0.21  0.7	2.77  2	3.32  3	0.60  1	0	30  20	72  5	60  1	4.7  0.5

Ammannia pedicellata 'Gold'

	% mg/ℓ							ppm µg/ℓ
Tank	C CO₂	N NO₃	P H₂PO₄	K	Ca	Mg	Na	Fe DTPA	Mn EDTA	Zn EDTA	Cu EDTA
8.3 pH 4.9	44.15  10	2.34  5	0.23  0.7	1.65  2	1.42  3	0.72  1	0	99  40	79  10	33  2	8.2  1
8.7 pH 4.8	44.60  10	2.55  5	0.24  0.7	1.77  2	1.33  3	0.54  1	0	75  80	107  20	43  4	7.6  2
8.8 pH 4.7	44.08  10	2.59  5	0.25  0.7	1.76  2	1.35  3	0.52  1	0	98  160	215  40	61  8	7.8  4

Rotala wallichii

	% mg/ℓ							ppm µg/ℓ
Tank	C CO₂	N NO₃	P H₂PO₄	K	Ca	Mg	Na	Fe DTPA	Mn EDTA	Zn EDTA	Cu EDTA
8.1 pH 5.3	42.28  10	2.35  15	0.50  2	2.52  4	1.41  3	0.61  1	0	69  40	24  10	29  2	12.0  1
8.3 pH 4.9	42.45  10	1.62  5	0.25  0.7	1.72  2	1.48  3	0.69  1	0	51  40	29  10	23  2	10.7  1
8.8 pH 4.7	43.27  10	1.74  5	0.31  0.7	1.71  2	1.24  3	0.52  1	0	63  160	65  40	47  8	13.1  4

🌱 Ammannia pedicellata 'Gold'

• Higher external concentrations of microelements led to slightly higher nitrogen (N) consumption and significantly higher manganese (Mn) and zinc (Zn) consumption.
  • The lower degree of orange-pink coloration of the growing tips therefore seems to be more a result of a slightly higher concentration of nitrogen (N) in the tissue than a higher concentration of iron (Fe), as I originally assumed.
• Interestingly, despite the increasing concentration of iron (Fe) in the water column, there was no adequate increase in its concentration in plant tissue (in other words, the concentration of iron in plant tissue remained more or less the same/stable despite its different external concentration).
• Given that virtually all of the tested recipes (with the exception of recipes #1 and #5) resulted in sufficient nutrient content in the plant tissue, it can be assumed that they are more or less optimal.
  • Soft-water, oligotrophic species​
    • bicarbonate (HCO₃) sensitive → prefers lower pH (4.5-5.5)
    • nitrogen (N) and/or phosphorus (P) sensitive → seems to prefer lower NP (3-5 mg/ℓ NO₃)
    • organic substrate sensitive → seems to prefer rather nutrient poor sediments (meaning that new organic substrates like ADA Amazonia can be detrimental)
    • benefits significantly from elevated CO₂ concentration (~10 mg/ℓ)
    • orange-pink discoloration of growing tips is a sign of iron (Fe) deficiency ... optimal nutrition results in leaves that are more yellow in color (with less orange tones)

🌱 Rotala wallichii

• Due to the rapid growth of this plant, it is important to ensure a regular/sufficient supply of nitrogen (N) and iron (Fe), which it can extract from the water relatively quickly. A concentration of 5 mg/ℓ NO₃ is sufficient for it [under the given conditions] only if it is renewed regularly (i.e., once a week). A concentration of 5 mg/ℓ NO₃ for 14 days seems to be insufficient for it [under the given conditions => light, CO₂, etc.].
• Higher concentrations of nitrogen (N) apparently lead to a richer red coloration of the leaves, but the concentrations of other nutrients (especially phosphorus and iron) must also be increased accordingly.
• Insufficient iron (Fe) concentration leads to a pinker coloration of the growing tips. The pink color is therefore a sign of iron deficiency. Conversely, the yellow color is a sign of nitrogen deficiency.
• R. wallichii does not appear to have a narrow spectrum of optimal nutrient concentrations. Although it can get by [under given conditions] with 5 mg/ℓ NO₃ and 40 µg/ℓ Fe, it can also easily handle higher nutrient concentrations (in previous experiments, it thrived even at 30 mg/ℓ NO₃). I think it is only important to maintain an appropriate N : P : Fe ratio (i.e., the more N it has, the more P and Fe it needs).
  • Soft-water, oligotrophic species​
    • bicarbonate (HCO₃) sensitive → prefers lower pH (4.5-5.5)
    • nitrogen (N) and/or phosphorus (P) tolerant → but with higher NP it seems to need higher µicro also
    • organic substrate sensitive → seems to prefer rather nutrient poor sediments (meaning that new organic substrates like ADA Amazonia can be detrimental) ... but I'm not sure what exactly bothers her about it
    • undemanding CO₂ user → can cope with naturally low CO₂ levels (~3-5 mg/ℓ)
    • pink discoloration of growing tips is a sign of iron (Fe) deficiency, while yellow discoloration of growing tips is a sign of nitrogen (N) deficiency ... optimal nutrition results in leaves that are more reddish-brown in color (with no pink or yellow tones)

🌱 Hygrophila corymbosa

• It seems to have a wide range of tolerance to nutrient concentrations, provided that they are somehow balanced (i.e., the more N it has, the more P and Fe it needs ... similar to R.wallichii).
• It grows well even at 3 mg/ℓ NO₃ and 0.3 mg/ℓ PO₄, but slightly higher nitrogen concentrations are probably better for it. It needs only relatively small amounts of potassium, calcium, and magnesium (similar to other species), but probably prefers more iron.
  • Hard-water, eutrophic species​​
    • bicarbonate (HCO₃) tolerant → but can get by with lower pH also (4.5-5.5)
    • nitrogen (N) and/or phosphorus (P) tolerant → but with higher NP it seems to need higher µicro also
      • at too low NP concentrations (~3 mg/ℓ NO₃), it forms long, thin capillary roots that can grow out of the substrate
    • organic substrate tolerant → higher nutrient content does not seem to cause any problems
    • benefits significantly from elevated CO₂ concentration (~10 mg/ℓ)
    • µicro tolerant
    • pink discoloration between the veins of the top leaves is a sign of iron (Fe) deficiency ... optimal nutrition results in leaves that are more green in color (with less red tones)

🌱 Alternanthera reineckii

• This plant seems to prefer root nutrition, but that doesn't mean it can't grow well in an inert substrate with the right water composition. [So far] it seems to do slightly better with higher iron concentrations. It appears to need little nitrogen and phosphorus (3-0.3 mg/ℓ NO₃ : PO₄). Subjectively, it did better at higher calcium concentrations (7 mg/ℓ Ca) than at lower ones (3 mg/ℓ Ca), but other factors may have been at play. I have not yet found the optimal recipe for this plant, so I intend to study it further.

PS: Please keep in mind that my comments apply to aquariums with lighting of 100-230 µmol PAR, 10 mg/ℓ CO₂, pH ~5, weekly changes of 50% water, etc.

Furhter notes:

• I see a clear and strong correlation between higher concentrations of NPµ (nitrogen, phosphorus, iron) and higher levels of algae/bacteria in my tanks.
  • For example, in an aquarium with 15 mg/ℓ NO₃ and 2 mg/ℓ PO₄, the occurrence of algae after one month was incomparably higher than in aquariums with 5 mg/ℓ NO₃ and 0.7 mg/ℓ PO₄.
  • It is possible that the increased concentration of nutrients is also largely related to the increased production of dissolved organic matter from the plants themselves (in other words, the more nutrients, the greater the amount of organic waste produced and excreted by plants). If you remove algae eaters, biological filtration, and all substrates that can "trap" dissolved organic matter (or excess nutrients) from the aquarium, you cannot fail to notice this correlation.
  • I believe that in aquariums with a sufficient number of algae eaters (typically shrimps) and powerful biological filtration, most aquarists will not notice this correlation, because shrimps and filtration can significantly reduce the problem of algae and bacteria. However, this in no way disproves the validity of this correlation.
• Pure silica sand appears to be a suitable substrate for growing most aquarium plants (including sensitive ones), provided that the aquarist uses an optimally balanced nutrient solution. However, for some plant species, such a solution has yet to be found.