Tool
Aquarium Stocking Checker
Assess a proposed or existing goldfish setup: the fish you keep, the water volume, and the filter. Get a clear verdict with the reasoning behind it.
What the verdict means
Excellent means the tank meets the comfortable figure for the fish in it, with a margin for growth and the odd missed water change. Adequate means it clears the minimum but leaves little headroom, so maintenance does more of the work. Needs improvement means the tank is below the minimum most experienced keepers would use. Too small means water quality cannot realistically be kept stable, and the fish are being held back.
Filtration can move the verdict. A tank with generous volume but a weak filter is downgraded, because goldfish produce a lot of waste and biological filtration is what converts it. A large sponge filter added alongside an existing filter is the cheapest way to add capacity.
What to do with each verdict
Excellent or adequate: nothing, beyond keeping the routine. If the verdict is adequate and you were thinking of adding a fish, the answer is that the tank has no headroom for one.
Needs improvement: work down this list in order, because it is ordered by how much each actually changes.
- More water. The only lever that fixes the cause rather than the symptom, and a food-grade tub costs a fraction of glass — the tank buying guide makes that case.
- Fewer fish. Immediate, effective, and covered by rehoming a goldfish, which sets out where they can actually go.
- More biological filtration. A large sponge filter on an air pump alongside the existing filter is the cheapest real capacity you can add.
- Bigger or more frequent water changes. This compensates rather than fixes, but it works, and the water change guide covers doing them without stressing the fish.
- Feed less. Least powerful, easiest to do today, and the one most people try first — overfeeding covers it.
Too small: the first two items are the only ones that matter, and the rest are delaying tactics.
Check the verdict against your own water
A calculator reasons from averages; your tank has a number. Test nitrate at the end of the week, immediately before a water change, and read it as the real verdict:
- Comfortably under 40 mg/L, stable month on month — the tank is keeping up, whatever this tool says.
- Climbing across weeks — the stocking is beyond what the routine clears, which is the definition of overstocked.
- Ammonia or nitrite above zero, ever — the biological capacity is short, and no verdict matters more than that reading.
Testing goldfish water covers taking a sample worth testing, and the parameter checker will interpret the numbers once you have them.
What the checker cannot see
Who eats first. A mixed-speed tank can pass on volume and still starve its slowest fish over months — chasing and bullying covers a failure that involves no conflict at all.
Footprint. Two tanks of the same volume are not equivalent if one is tall and one is long; goldfish use floor area and surface, not height.
Whether the filter is cycled. A perfectly stocked tank with an uncycled filter is dangerous today, and cycling is the page for that rather than this tool.
The fish themselves. A breed with poor swimming or poor sight needs more from its tank than its volume requirement suggests, which the individual breed pages cover.
The assumptions
The checker uses the same working ranges as the rest of the site: 75 to 100 litres for a first fancy goldfish and 40 to 50 for each additional one; 150 to 200 litres for a first single-tail and 75 to 100 for each additional. It assumes adult size, because goldfish do reach it in decent conditions, and it assumes a weekly water-change routine. If you keep an unfiltered bowl, the answer is the same regardless of the numbers: the bowl guide explains why, without a lecture.
Frequently asked questions
The checker says "Needs improvement" but my fish seem fine. Is it wrong?
Goldfish are tough and can look fine for a long time in conditions that are slowly wearing them down: stunted growth, a weakened immune system, chronic nitrate. The verdict describes headroom, not the fish's mood today. If you are keeping up with large water changes, you are compensating with effort for what volume would give you for free.
Why does it mark down a mixed tank of fancies and single-tails?
Speed. Common and comet goldfish reach food first and bump slow, round-bodied fish out of the way, so fancies in a mixed tank are often chronically underfed and stressed. Kept in separate groups, both do better.
The verdict got worse and I did not add a fish. Why?
Because something else in the entry changed. Usually it is the volume: a tank sold as 200 litres holds noticeably less water once the substrate, the decor and the gap under the lid are taken out, and entering the real figure from the volume calculator lowers the verdict. The others are a fish moved between the two groups, since a single-tail needs about twice the room of a fancy, a filter flow entered for the first time, or US gallons chosen for a tank measured in UK ones. The checker already assumes adult size, so the fish growing up does not change the verdict; it only changes how soon the tank shows it.
How do I actually confirm the verdict on my own tank?
Test nitrate at the end of the week, immediately before a water change. If a normal weekly change brings it comfortably under 40 mg/L and it does not creep upward month on month, the tank is keeping up whatever the verdict says. If nitrate is climbing across weeks, or ammonia or nitrite ever read above zero, it is not — and that reading outranks any calculator, including this one.
Do snails, plants or other tankmates count toward the stocking?
Snails add a little waste and are usually a rounding error next to a goldfish. Plants help slightly with nitrate and are nowhere near a substitute for water changes. Other fish count fully, and in a goldfish tank they usually cost more than their volume suggests, because temperature and speed mismatches do damage that has nothing to do with stocking density.
What flow rate should I enter?
The rated flow printed on the filter, in litres or gallons per hour. Many keepers work to a rule of thumb of about five to ten times the tank volume per hour, the figure the filtration guides use. Real flow drops as media clogs, so the checker treats anything below about four times the tank volume per hour as low and six times or more as healthy for goldfish.