If you want Malawi mbunas then they'll be the only fish in the tank. You need masses of rockwork to give them hiding places and would be looking at around 20-30 fish in the 260 or 30-40 in the 450 depending on which species you want to keep.

That's still higher than it should be - consider using Pond Prime to protect the fish from its effects. Double dose daily until the spike in nitrite resolves back to 0.

Hmm - very inventive, but a tad small for Koi and perch.

Hi

He's going to need a bigger tank and an external canister filter in the long term, but for now, this is a good option: https://www.ebay.co.uk/itm/EHEIM-PICKU ... m:mVKrBUHrJnEvmdpcbcn4u9g

Please read this article about ammonia and nitrite spikes as in a new tank, until the filter becomes established he will be subject to those which will harm and potentially kill him: https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm

I agree, looks around 80 or so which may well be the cause of the problem. Test your tap water nitrate level too as if it's high (mine is) it will be much harder to get the tank down to a suitable level and you will need to consider other options for doing so such as the use of RO water or nitrate absorbing filtration media.

What's your nitrate reading? Sometimes, particularly as they age, goldies can have swim bladder issues triggered by nitrate in the water and you need to keep it pretty low for them (ideally under 20ppm).

OK. You need a decent liquid based test kit like this one: https://www.ebay.co.uk/itm/API-MASTER- ... m:mus6ISSe6hh_Tugj3Fs3zhA

However, 30 litre is extremely small for a goldfish and he looks stunted - at 2 years old he should be a very large, 8-inch long fish (excluding tail). He is most likely suffering from poisoning from excess nitrate due to the tank being too small and unfortunately when they get to the stage of bending into a C shape like that there is often very little chance of recovery. You can try though by changing most of his tank water - BUT in small stages, a bit at a time over a couple of days and make sure to use a dechlorinator on the new water.

If he recovers, make sure when you put him in his new tank that you move his existing filter over too or he will be at risk of ammonia and nitrite poisoning as the new tank and filter won't have the beneficial bacteria that convert his waste into less harmful nitrate (which you will control by doing a regular weekly partial water change of at least 25%)..

Neither is easier; the only real difference is you need a heater for tropicals. Tropicals give you far more choice though as there are very few small cold water fish that are suitable for a tank of that size (for instance it's much too small for goldfish). You don't need an air pump as long as you have a good filter with its return disturbing the surface. Live plants aren't essential.

The nitrite reading is dangerously high. You need to change a lot of water to get that down to 0. Use a conditioner called Pond Prime in a double dose which will protect the fish from the effects of the nitrite for 24 hours between water changes.

Can you tell us what your test results show for ammonia, nitrite and nitrate, please? Also how big is the tank, how long has he been in it and are there any other fish in there with him?