Compatibility Checker

How compatibility is judged

Each species is assessed on the things that actually go wrong in mixed tanks: whether its temperature range overlaps with the 18 to 24 °C fancies prefer or the wider 10 to 24 °C single-tails tolerate; whether one animal can eat, injure or nip the other as they grow; whether it competes for the same food at a different speed; and whether it needs a current, a diet or a water chemistry that goldfish do not. Where the answer differs for slow, round-bodied fancy goldfish and fast single-tails, the checker gives a separate verdict for each.

Read the verdict as a starting point

"Generally compatible" means the combination usually works in a tank sized for everyone in it. "Possible, with caution" means it can work under specific conditions, which the result lists. "Not recommended" means the risks outweigh the appeal, however often the combination is sold in shops. Individual animals vary, so introduce any new tankmate with a plan to separate them if it goes wrong, and quarantine new arrivals for at least four weeks first.

Why mixed tanks fail, and it is often not predation

The picture people have is a goldfish eating something. That does happen — goldfish will take anything that fits in a mouth, and the mouth gets large — but it is often not the main cause.

1. Speed at feeding time. The quiet one, and often the most damaging. A faster animal does not need to be aggressive to starve a slower one; it simply arrives first, at every meal, for a year. Chasing and bullying covers a failure in which nothing ever looks wrong until a fish is thin.
2. Temperature. Most tropical species want 24 to 28 °C, which is above where goldfish do well long term. A tank held between the two suits neither, and the compromise is usually paid by whichever animal is less hardy.
3. Adult size. Compatibility at 5 cm says nothing about compatibility at 25. This is why the checker sizes for the adult, and why shop advice and this tool often disagree — they are describing different fish.
4. Fins and eyes. Nippy species take long fins as a target of opportunity, and a bubble eye or telescope offers something fragile that no other fish is carrying.

The combinations people ask about most

Tankmate	Verdict	The reason
Another goldfish, same type	Best	Same water, same food, same speed
Fancy with single-tail	Avoid	Speed, at every meal, for years
Koi	Pond only	Reaches 60–90 cm and needs thousands of litres
White cloud mountain minnows	Caution	Temperature overlaps; they are mouth-sized
Nerite and ramshorn snails	Usually fine	Hard shells, useful on algae
Shrimp	Avoid	Exactly the size and colour of a treat
Plecos	Avoid	Too warm, too large, and they rasp at slime coats
Axolotls and turtles	Avoid	Different temperature, and one eats the other

The checker holds the full list with the conditions attached, and goldfish tankmates works through the reasoning at length — including the case that ends most of these discussions, which is that a goldfish-only tank is not a compromise.

If you decide to try it

Quarantine the newcomer for at least four weeks, without exception — quarantining a new goldfish applies to every species, not only to fish.

Some keepers rearrange the decor on the day, so the residents are re-learning the tank while they meet the newcomer, though goldfish hold little territory and it is optional. Add it at a feeding time when everyone is occupied.

Then judge it at two weeks, not two days. A fortnight of sorting out is normal. What is not normal is an animal that never starts eating, accumulates damage rather than healing, or has been singled out by one resident that does nothing else.

And have somewhere for it to go before you start, because "it did not work out" is only a manageable outcome if there is a second tank or a plan.