What Fe+ is for — DSR Fe+ is used mainly as a phosphate binder; the trace-element role is minor. Active iron binds phosphate and precipitates (slight haze); brown/rust skimmate and darker skimmate water are normal. Prefer dosing late evening (~23:00) because of possible haze; keep your skimmer running well.

Steering phosphate — aim often within 0.02–0.10 ppm. Below ~0.02 ppm can bleach corals; above ~0.12 ppm can cause brown corals and tissue loss (sensitive Acropora may shed tissue). Rock and substrate can release PO4 slowly (buffering) and absorb phosphate as well (especially porous rock), so measured PO4 can stay high or “jump”; a one-off PO4 removal rarely fixes that — gradual Fe+ dosing also binds phosphate that leaches from the rock over time.

Daily dosing limits — at most 0.15 ppm Fe+ per single dose; measured iron should not go above ~0.25 ppm. Up to ~0.24 ppm PO4 is often enough with once daily 0.15 ppm Fe+; above ~0.36 ppm you may need twice daily (12 h interval). The chelate also acts as a carbon source and can affect NO₃/PO₄ — keep nitrate stable if needed (e.g. 1–2.5 ppm).

Normal build — day 1: 0.03, day 2: 0.06, day 3: 0.09, day 4: 0.12, day 5+: 0.15 ppm/day (20/40/60/80/100% of the per-dose maximum).

Accelerated (very high PO4, > 0.64 ppm): twice per ~12 h following the DSR step plan in this calculator; when PO4 has dropped to about 0.10 ppm or into the 0.02–0.10 ppm steering band, in practice go back to once daily.

Warning — some tangs, angelfish, butterflyfish and Tridacna spp. may be sensitive to iron overdose; ramp slowly. Fe+ is mostly not measurable after ~4–8 h; if you check dosing, ~20 min after dosing is suggested. Iron is hard to keep perfectly “flat”; the main goal of testing is to avoid harmful build-up.