Aquatic plants fail in a small number of ways, and almost all of them trace back to one input being out of proportion with the other two. Reading the symptoms is a matter of knowing what each deficiency looks like.
The three inputs, and why proportion matters
Photosynthesis consumes light, carbon dioxide and water, and uses nutrients — nitrogen, phosphorus, potassium and trace elements — to build tissue. If one input is short, growth is limited by it and the others accumulate unused. Unused light and unused nutrients in the water column are exactly the conditions algae need.
This is why raising light is the most common cause of an algae problem. It is also why the correct response to algae is almost never "more light" and almost always "more CO₂ and nutrients, or less light".
| Input | Typical supply | Limiting when | Raise by |
|---|---|---|---|
| Light | 20–150 µmol/m²/s | Plants stretch, lower leaves drop | Higher fixture or longer photoperiod |
| CO₂ | 2–30 ppm | Stunted growth, algae despite nutrients | Injection, or better flow |
| Nitrogen | 5–20 ppm NO₃ | Old leaves yellow uniformly | Nitrate dosing |
| Potassium | 10–30 ppm K | Pinholes, black spots, yellow between veins | Potassium supplement |
| Phosphate | 1–3 ppm PO₄ | Stunted tips, purple undersides | Phosphate dosing |
| Iron | 0.1–0.5 ppm | New leaves pale, veins stay green | Trace / iron supplement |
Reading the symptoms
Lower leaves yellow and drop, new growth is fine
The classic sign of a mobile nutrient deficiency — nitrogen or potassium. Plants move these from old tissue to new, so the old leaves suffer first. Nitrogen deficiency yellows uniformly from the leaf tip backward; potassium deficiency shows as pinholes and yellowing between the veins, often with black spots on plants like Java fern.
New leaves are pale while old leaves stay green
An immobile nutrient deficiency — iron, calcium or magnesium. Plants cannot relocate these, so the newest growth shows it first. Iron deficiency pales the leaf between the veins while the veins stay dark. Calcium deficiency distorts and curls the new leaf edges.
Plants stretch with long internodes and small leaves
Light is limiting. The plant is investing in stem length to reach more light. Increase intensity or reduce the distance between fixture and water — and note that this is the opposite problem from CO₂ limitation, which produces compact but stunted growth.
Compact growth but slow, with algae on the leaves
CO₂ is limiting. Light is adequate and nutrients may be adequate, but there is not enough carbon to use them. This is the single most common presentation in a medium-light tank without injection. Increase flow so existing CO₂ reaches the leaves before raising the dose.
Fertilisation regimes
Two schools dominate. The Estimative Index doses everything generously, in excess, and relies on a large weekly water change to reset the levels. It is simple and it removes nutrient limitation entirely as a variable. Lean dosing supplies just enough, relying on plant uptake to keep levels low, which reduces algae substrate but demands more precision.
- EI: dose nitrate, phosphate, potassium and traces separately, six days a week, 50% water change weekly. Forgiving and effective.
- Lean: dose a fraction of EI, targeting 5–10 ppm nitrate. Slower growth, less margin for error.
- Liquid carbon: not a CO₂ substitute. Glutaraldehyde-based products suppress algae directly and supply a small amount of usable carbon. Useful in low-tech tanks, harmful to some plants in quantity.
- Root tabs: for inert substrates only. Push a capsule beside heavy feeders every 8–12 weeks.
Trimming, and why it matters more than dosing
Trimming is not cosmetic. A stem plant that reaches the surface shades everything below it, and a carpet that has grown 5 cm thick is blocking light from its own base. Both cause the bottom of the tank to fail while the top looks fine.
Stem plants
Cut the top third off and replant the cutting beside the original. This produces a denser group and resets the height. Discard the leggy lower stem once it stops producing side shoots.
Carpets
Trim in a shallow arc with curved scissors. A flat cut leaves a scorched brown surface for a week; an arc hides the cut edges.
Rosettes and epiphytes
Remove the oldest outer leaves at the base. Do not cut into the rhizome or the crown.
Remove every cutting
Floating fragments root in the filter intake, in the carpet and against the glass. Net them out immediately.