Reef Potassium Dosing Guide: Symptoms and Target Levels
Potassium is the quiet major ion that rarely makes headlines until your red Montipora turns brown and your chalices fade to pastel. This reef potassium dosing guide from Gensou Aquascaping in Singapore lays out how the element behaves, why natural seawater values matter, and how to correct a deficit without wrecking your ionic balance. Twenty years of keeping SPS in a humid tropical climate has taught us that potassium, not magnesium, is the element most hobbyists overlook.
Natural Seawater Baseline
Natural seawater holds potassium at approximately 400 ppm. Most commercial salt mixes deliver 380-420 ppm fresh from the bucket, but Red Sea Coral Pro and Tropic Marin Pro Reef can test 10-15 ppm low out of the bag. Always batch test a fresh mix before assuming your tank has drifted.
Why Potassium Matters
Potassium participates in zooxanthellae photosynthesis and red pigment expression in corals. Deficiency first shows in red pigmented Acropora and Montipora capricornis, which lose saturation and develop a washed-out orange tint. Fish use potassium in osmoregulation, though fish-driven depletion is negligible compared with coral uptake.
Target Range and Testing
Aim for 380-420 ppm. Salifert and Red Sea kits give useful hobby-grade readings to within 10 ppm. Test monthly if you actively dose; quarterly is enough otherwise. ICP-OES remains the gold standard – see our reef tank icp oes test interpretation article for how to read those results.
Symptoms of Deficiency
Loss of red and pink coloration is the classic marker. Pocillopora damicornis browning, Montipora setosa fading and chalice corals losing eyes all point toward potassium below 360 ppm. Tissue recession at the base of staghorn Acropora sometimes follows, though alkalinity swings cause the same symptom and need ruling out first.
Sources of Loss
Heavy GFO use strips potassium alongside phosphate. Zeovit and carbon dosing systems export potassium through bacterial skimming. Water changes with a low-potassium salt mix gradually drift the tank down even without uptake. Refugiums running Chaetomorpha pull roughly 0.3 ppm potassium per gram of dry macroalgae harvested.
Calculating a Dose
Potassium chloride (KCl) is the standard hobbyist source. One gram of KCl in 100 litres raises potassium by about 5.2 ppm. A tank testing 360 ppm with 200 litres net water volume needs roughly 15 g of KCl to reach 400 ppm. Split the correction over five days to avoid jolting fish or shrimp.
Potassium Chloride vs Sulphate
KCl is cheap, available at aquarium shops for around $8 SGD per 500 g, and perfectly safe up to 450 ppm. Potassium sulphate costs more and adds sulphate that your salt already provides adequately. Stick with KCl unless your ICP report specifically flags chloride creep.
Building a Maintenance Regime
Once corrected, most mixed reefs need 0.5-1 ml of 10% KCl solution per 100 litres daily to hold 400 ppm. SPS-dominant tanks with aggressive growth can need double that. Add the dose through your existing dosing pump; see our best dosing pumps reef tanks overview for hardware options.
Interactions with Other Elements
Potassium chloride adds chloride that slightly lifts salinity. At the dose rates above the change is undetectable, but if you push potassium above 450 ppm by mistake, perform small water changes rather than trying to remove chloride selectively. Excess potassium suppresses strontium uptake and can dull SPS polyp extension.
Troubleshooting Pale Corals
If red pigment fails to return after two weeks at 400 ppm, look at light intensity, nitrate and iron before dosing more potassium. A tank running nitrate below 1 ppm will refuse to express red regardless of potassium. Pair this guide with our reef trace elements dosing guide for the full coloration picture.
Singapore Sourcing
Reef-grade KCl is stocked at shops along Pasir Ris Farmway and the Clementi aquarium cluster. Food-grade KCl from NTUC baking aisles is equally pure and often cheaper. Store it dry; Singapore humidity cakes the powder within weeks.
Related Reading
emilynakatani
Still Have Questions About Your Tank?
Drop by Gensou Aquascaping — most walk-in questions get answered in under 10 minutes by someone who has set up hundreds of tanks.
5 Everton Park #01-34B, Singapore 080005 · Open daily 11am – 8pm
