Established tank slowly dying out

[Waxlyrically]

Help. I have had a tank for the last 10 years with mainly rainbow fish in it. About a year ago I introduced 2 red rainbow fish which died and since this the existing fish many of which I had for years have been slowly wasting away. They get progressively thinner and then suddenly die. I Only have 3 left now but don't want to restock if the red rainbows introduced some horrible disease. Not sure what happens to the filter if you deplete the tank totally though. Is it a complete clean and recycle? Or should I start adding a few new fish of a different species? Wondering if rainbows are now being overbred and subject to nasties which I have inadvertently introduced. So sad for my poor fish - any advice welcome.

Aw no, it's terrible when something like this happens :( Did you quarantine the new fish? It sounds like they may have brought something in. Have you tried treating them? Wasting away can be a symptom of a few things, are there any other symptoms at all?

If all the stock is dying off then I would be tempted to strip everything down, give it a good clean and start over with new filter media, so you'd have to start the cycle from scratch.

I haven't heard anything about rainbows being overbred, but the general quality of fish has declined (I think) and even the good fish shops are under a lot of pressure to turn a profit. So you're more likely to end up with something weak and/or infected with something.

No quarantining I'm afraid as I don't have a second tank - so I guess this is the problem. The place I bought them claim to quarantine all their fish but I'm sure this isn't foolproof.

The fish seem to lose colour, then begin to look concave but they feed and act normally for many more weeks before suddenly dying. The only comforting thing is they don't appear to suffer unduly with no signs of behaviour change at all. I'm not sure what to treat as my book suggests fish TB and recommends taking to the vet!

They are all quite old around 8-9 years so I suppose for fish they've not had a bad innings. I will wait and see what happens - it's possible the last few might survive (none looking poorly at the moment). If they survive I will gradually restock if not it's as you say a clean and complete new start up.

Might need to invest in some sort of quarentine tank though. How big do these have to be? Assume needs cycled filter etc in the normal way as they stay in a while. How do you sustain the filter while its empty?

Sorry for all the questions.

TB is one of the things that can cause wasting, so I would be cautious about sticking your hands in the water, but there are other things that can cause it too. You could try treating with a broad range treatment like Melafix and see if that helps any. 8-9 years is a pretty good run so it might be that age has weakened their immune system.

Q tanks just need to be big enough to hold the fish you are quarantining for a few weeks. With small fish you could go for 40 litres, bigger fish I would say 60+. Mine is just a bare tank with plastic plants so easy enough to drain and pack away. With the filter, you can either just take half of the sponge from your mature filter or keep a small sponge filter running in the main tank and just move it across when you need it.

Even if you do quarantine, some things can still slip in. But more visible stuff like whitespot and fish lice can be spotted.