Coralkin honest saltwater & reef care

Free tool

Reef stocking & tank-size calculator

The most expensive beginner mistake is too many fish in too little water. Tell us your tank and your wish-list — we'll check each fish against its fact-checked minimum tank and shoal size, flag aggression and reef-safety clashes, and give you an honest read on how full the tank is.

1 · Your tank
…or work it out from tank dimensions (inches)

A rough guide — real filled volume is a little less than the glass dimensions (rock and sand displace water too).

2 · Your fish

Only fish with a fact-checked Coralkin care guide appear here — more as we source them.

An honest tool, not a promise. The minimum-tank, shoal-size and compatibility checks come straight from our fact-checked care guides. The "stocking level" gauge is a deliberately conservative, rough estimate — marine fish need far more water per inch than freshwater, and real capacity depends on your skimmer, filtration, flow and water changes. The only true judge is your water: test your nitrate and phosphate. For the full pairwise picture use the fish compatibility checker, and check your numbers against the reef parameter targets.

Adult size & minimum tank, per fish

Plan for the adult, not the juvenile in the shop. Every figure is from that fish's fact-checked care guide — shortlist here, then run your wish-list through the calculator above for the honest capacity read on your tank.

FishAdult sizeMinimum tankNormally kept
Banggai Cardinalfish ~3 in 20 gal One or a pair
Bicolor Blenny ~4 in 10 gal One
Blue Tang ~10 in 100 gal One
Coral Beauty Angelfish ~4 in 70 gal One
Firefish Goby ~3 in 20 gal One or a pair
Flame Angelfish ~4 in 70 gal One
Flame Hawkfish ~3.5 in 30 gal One
Green Chromis ~3 in 30 gal Group of 6+
Green Clown Goby ~1.5 in 10 gal One or a pair
Kole Tang ~6 in 70 gal One
Lawnmower Blenny ~4.5 in 30 gal One
Lyretail Anthias ~4 in 70 gal Group of 3+
Mandarin Dragonet ~3 in 30 gal One
Melanurus Wrasse ~5.5 in 55 gal One
Midas Blenny ~4.5 in 30 gal One
Neon Goby ~2 in 10 gal One or a pair
Ocellaris Clownfish ~3.5 in 15 gal One or a pair
Orchid Dottyback ~3 in 20 gal One
Pajama Cardinalfish ~3 in 20 gal Group of 5+
Percula Clownfish ~3 in 15 gal One or a pair
Powder Blue Tang ~9 in 125 gal One
Royal Gramma ~3 in 20 gal One
Six Line Wrasse ~3.5 in 30 gal One
Tailspot Blenny ~2.5 in 10 gal One
Yellow Tang ~8 in 90 gal One
Yellow Watchman Goby ~3.5 in 20 gal One or a pair

Adult sizes are representative "plan-for-the-adult" figures from each care guide; real capacity depends on filtration and your nitrate/phosphate test, not inches alone.

How many fish can my reef tank hold?

How many fish can I put in a saltwater tank?

Far fewer than a freshwater tank — marine fish need much more water per inch. As a conservative rule of thumb we use about half an inch of adult fish length per US gallon, then let your nitrate and phosphate tests set the real ceiling. Crucially, plan for the ADULT size, not the juvenile you buy: a "small" yellow tang reaches 8 inches. Enter your tank and wish-list above for an honest, per-fish read.

How many fish in a 20 gallon nano reef tank?

Roughly 2–3 small, peaceful fish (about 10 adult inches at our conservative marine budget) — think a pair of clownfish plus a small goby or blenny. Skip tangs, dwarf angels and large wrasses; they need 55–125 gallons. The real limit is bioload, so keep nitrate and phosphate low and add fish gradually.

How many fish in a 40 gallon reef tank?

Roughly 4–6 small fish (about 20 adult inches at our conservative marine budget) — for example a clownfish pair, a goby, a blenny and a small schooler. Still no tangs (a yellow tang needs 90 gallons). Filtration, skimming and water changes shift the real number, so let your test kit have the final say.

How many fish in a 75 gallon reef tank?

Roughly 8–10 small fish (about 37 adult inches at our conservative marine budget), or a single tang plus a handful of small fish. A 75 opens the door to a kole or yellow tang, one dwarf angel and a wrasse — just watch the same-group rules (one tang, one dwarf angel). Test nitrate and phosphate to confirm your real capacity.

Do I need to plan for a fish’s adult size?

Yes — always stock for the adult, not the inch-long juvenile in the shop. Overstocking is the most expensive beginner mistake because the tank looks fine for months, then the growing fish push nitrate up and aggression with it. This calculator uses each fish's cited adult size and minimum tank so the plan holds a year from now, not just on day one.