Reef chemistry, without the folklore.
Thirty-five calculators for dosing, salinity, light, flow and tank volume. Each one shows its formula and where every constant came from, so you can check the work instead of trusting a forum post.
Free. No account. The math runs in your browser.
What in range looks like on your test card.
Each tile is the colour your kit shows inside the site's target band. Titration kits show the change at the endpoint. Colours approximate common kits, so read against your own card.
- Nitrate2–10NO₃ ppm · Pink at 5 ppm on a colorimetric card
- Phosphate0.02–0.10PO₄ ppm · Pale blue at 0.05 ppm
- pH7.9–8.4pH · Orange-red at 8.2 on an indicator card
- Salinity35ppt · Refractometer line at 35 ppt, 1.0264 SG
- Alkalinity7–11dKH · Titration: blue until the pink endpoint
- Calcium380–450ppm · Titration: pink until the blue endpoint
- Magnesium1,250–1,400ppm · Titration: violet until the blue endpoint
- Temperature76–81°F · Hold within half a degree of 78
All 35 calculators
Sorted by what you are holding: a test result, a tape measure, or a box of equipment.
Test result in hand18
Tape measure in hand7
Between 6 and 24 inches down, three-quarters of the light is gone.
Measured PAR straight under an AI Hydra 32 HD at full power, from the site's database of 20 reef LEDs. The labels on the right are the PAR calculator's coral bands.
Compare all 20 fixturesWhere the numbers come from
- Randy Holmes-Farley. Two-part, magnesium and kalkwasser stock strengths and stoichiometry.
- Apogee Instruments. Correction factors for quantum sensors used underwater.
- NIST. Gallon, litre and water density definitions.
- Red Sea, Tropic Marin, Bulk Reef Supply. Published dose constants for their own products.
- CaribSea. Bulk densities for aragonite and crushed coral.
Guides
All 13- How to use and calibrate a saltwater refractometer8 min
- Starting a reef tank: the chemistry-first checklist9 min
- Two-part dosing, explained8 min
- Reef tank lighting: PAR, PUR, and the Apogee correction11 min
- Vinegar & vodka dosing calculator for reef tanks10 min
- ICP testing for reef tanks: ATI, Triton, Oceamo, ICP-Analysis10 min
Common tank sizes
All sizesQuick reference: real dimensions, calculated gallons, filled weight, and recommended glass thickness for the sizes hobbyists actually buy.
- Standard 10 gallon20 × 10 × 12 in · 10 US gal
- 20 gallon long30 × 12 × 12 in · 19 US gal
- Standard 29 gallon30 × 12 × 18 in · 28 US gal
- 40 gallon breeder36 × 18 × 16 in · 45 US gal
- Standard 55 gallon48 × 13 × 21 in · 57 US gal
- Standard 75 gallon48 × 18 × 21 in · 79 US gal
- 90 gallon48 × 18 × 24 in · 90 US gal
- 120 gallon48 × 24 × 24 in · 120 US gal
- 125 gallon72 × 18 × 21 in · 118 US gal
- 180 gallon72 × 24 × 25 in · 187 US gal
- Innovative Marine NUVO 30L24 × 18 × 16 in · 30 US gal
- Red Sea Reefer 25036 × 22 × 21 in · 72 US gal
More references
- Troubleshooting
Cloudy water, algae, crashing alk, dying coral. Likely causes ranked by how often they're the culprit, with the test that confirms each.
- LED PAR database
PAR at depth for 20 reef LED fixtures — AI Hydra, Radion XR15/XR30 G6, Kessil A360X, Orphek, ATI Straton hybrid, and more.
- What size tank for…
Adult-minimum tank sizes for 70+ freshwater and saltwater species — betta, oscar, clownfish, yellow tang. Cited sources.
- Parameter index
Alkalinity, calcium, magnesium, salinity, pH, NO₃, PO₄, temperature.
- Ideal reef parameters
What the experts actually disagree about.
Recently updated
Hobbyist staples
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The reef supplies every tank ends up with eventually. Test kits, glass cleaner, refractometer, salt, cycling bacteria. From the calculators above, these are what you'd buy first.
The hobbyist standard for alkalinity testing.
Calibrate with 35 ppt fluid monthly. Replaces hydrometers.
Monthly refractometer calibration. Skip and your salinity drifts.
The hobby standard magnetic algae scraper. Sizes for any tank.
Best per-gallon value for SPS tanks.
Seeds nitrifiers during cycling. Pairs with Ammonium Chloride.