Phosphate is one of the two main "nutrients" in reef tank chemistry (the other being nitrate). Sources: fish food, fish waste, decomposing organics, tap water (in some regions), some salt mixes (Reef Crystals slightly elevated). Reef target: 0.02-0.10 ppm with high-precision Hanna ULR phosphate checker. Salifert kits read coarsely above 0.05 ppm. Effects of high PO4: brown spotting algae on rocks, dinoflagellates, cyano outbreaks, SPS coral browning + slowed growth, reduced coral coloration. Effects of zero PO4: dinoflagellate blooms (paradoxically - they take over when other algae starve), STN/RTN risk for SPS, coral pale + bleached. Reduction methods: GFO (granular ferric oxide) in fluidized reactor; carbon dosing (vodka/vinegar/NoPoX); refugium with chaeto; aluminum-based phosphate binders (Phosguard - controversial). Increase methods: if tank reads 0.00 ppm and showing dino bloom - dose neophos or potassium phosphate to bring up to 0.05 ppm. Freshwater context: planted tanks WANT 1-3 ppm phosphate as plant fertilizer.
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
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