I have a 24" cube tank, which has a substrate of crushed coral (it used to house cichlids), a large piece of mopani wood and a couple of plastic plants. Filtration is via a Hydor Prime canister filter. I do 20%-30% water changes every week or two.

I put in four goldfish some months ago, which seemed to be doing OK at first. However, after a few weeks, one of them started sitting on the gravel and not moving much. After a day or so, it was dead. One by one, the others did the same. I bought some more, but the same thing happened. I'm down to one goldfish, which seems healthy enough.

I realise that it's a bit of a vague question, but can anyone advise as to why I can't seem to keep the goldfish alive? I would really like to buy more, but don't want to if they are going to die after a few weeks.

Thanks for any help