How risky is adding a companion goldfish?

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A reader is looking to add a companion for his Fantail goldfish, although he is nervous about adding a new fish and wants to do it safely. Jeremy Gay advises…

I have had my Fantail goldfish for five years and I am extremely fond of him. He started off with another Fantail which sadly died about four years ago after becoming tangled up in some weed —I now only use artificial ornaments in the tank. The remaining fish has been on his own ever since.

I feed him once a day with a pinch of flake food; he seems happy on this but should I be varying his diet or feeding him more often?

Should I get him a companion? I am very anxious about introducing a new fish as I worry about disease, but I don’t want him to be lonely. How could we introduce a tankmate safely?

KIM PALMER, VIA EMAIL

Jeremy replies: Feeding once a day is fine. A good brand of flake provides a complete diet, but most fancy goldfish keepers feed a sinking pellet as they believe it lessens the likelihood of swimbladder problems. Frozen food is also good for fancy goldfish, so you could try some frozen bloodworm every now and again.

Goldfish are social creatures, although looking at the photo you sent, the relatively small tank size does work against adding a companion, as the biological load will be doubled and together their growth may become stunted. If you can get a bigger tank that would be much better. Choose either another Fantail, or an Oranda, Ryukin or a telescope-eyed goldfish. You are right to worry about disease, however. The biggest risk with fancy goldfish is flukes, which are invisible to the naked eye, but deadly. So if you add another fish, I’d quarantine it for 4-6 weeks in a separate tank to make sure it hasn't brought any parasitic infections with it, just to be on the safe side.