What Plants Actually Need
Macronutrients: nitrogen (N), phosphorus (P), potassium (K). Plants need these in quantity.
Micronutrients: iron (Fe), magnesium (Mg), trace metals (Mn, Cu, Zn, B, Mo). Needed in tiny amounts.
Sources in a Stocked Tank
Fish food and waste provide most N and P. A moderately stocked tank usually has plenty of both.
K must usually be supplemented. Iron and traces can be supplemented or rely on fish food remainders.
Liquid Fertilizer Basics
Seachem Flourish Comprehensive: trace minerals + Fe + K. Dose 5 ml per 10 gallons weekly.
Seachem Flourish Potassium: K only. Dose if leaves develop pinholes (K deficiency).
API Leaf Zone: budget alternative. Slightly less concentrated.
Skip nitrate (Flourish Nitrogen) and phosphate (Flourish Phosphorus) supplements unless you have a heavily planted, lightly stocked tank.
Root Tabs
For root-feeding plants (swords, crypts, vals): one tab per plant cluster every 4–6 months.
API root tabs are budget choice. Seachem Flourish Tabs are higher quality.
Bury 1 inch deep near plant roots.
Deficiency Signs
Yellow new growth: iron deficiency. Add Flourish Iron weekly.
Pinholes in leaves: potassium deficiency. Add K supplement.
Small new leaves with pale veins: nitrogen deficiency (rare in stocked tanks). Reduce water changes or add a fish.
Stunted growth, dark spots: phosphate deficiency (rare). Add Flourish Phosphorus.
Overfertilization
Algae bloom is the first sign of over-dosing in low-tech tanks. Cut dosing by 50% and observe for 2 weeks.
Plant uptake in no-CO2 tanks is limited — extra nutrients feed algae, not plants.