Zinc (Zn)
A trace micronutrient plants need at low concentrations. Included in most complete trace mixes; toxic if substantially over-dosed.
What it is
Zinc is a plant micronutrient involved in enzyme regulation and in the synthesis of growth hormones (auxin). Required at low concentrations — typical aquarium target 0.05–0.1 mg/L — against a toxicity ceiling around 1 mg/L.
Why it matters
Zinc deficiency is uncommon in planted tanks that use any complete trace mix. The interesting case is the opposite: zinc contamination from galvanised plumbing fittings or zinc-coated tank lids, which can quietly elevate Zn beyond the toxicity threshold and cause unexplained livestock losses, particularly in shrimp.
How to test
JBL Zink or comparable kit. Hobby kits read in the 0.05–1.0 mg/L band; sub-0.05 readings need a Hanna ULR-style checker.
What high and low look like
Below 0.05 mg/L sustained, in an otherwise well-fertilised tank: stunted growth, particularly on apical shoots. Above 0.3 mg/L: shrimp moult failures, fish growth slows. Above 1 mg/L: acute toxicity for invertebrates.
How to fix
For deficiency: complete trace mix at standard dosing rates. For contamination: identify and remove the source (zinc-plated lid, galvanised pipe in the top-up reservoir), then RO water changes to dilute the existing burden. Zinc binds to substrate and decor and leaches back slowly — a single contamination event can take months of frequent water changes to fully clear.
