I have six goldfish, and one of them recently has started just floating on it's back. It has a large, significant lump all around it's stomach area. It has had this lump for just over a year now, but has only started floating upside down for about a fortnight. I sometimes fear it to be dead, until I see it's gills moving in and out fine, and blinking. It usually tries to become on it's correct side, and not being able too, but now just floats around the top of the tank blinking and turning around once in a while. Btw - I'm not planning to purposely kill it, flush it etc because I've had the fish for about seven years.

Any ideas what it could be?
Thanks.

If the fish is just floating it could be swim bladder disease which means that their bladders are inflated with air and they cannot deflate them, to be honest the best thing you could do is it put it out of its misery. What size is your aquarium and what type of filtration do you use?

Six is a lot of goldies.

Any chance of setting up a separate, filtered hospital tank using some cycled water from your existing tank, and treating it appropriately?

There are treatements for swim bladder but I'd start Googling fish diseases and diagnosis as it could be something else.

One goldie = 10 galls of water. They will start dying off if you have anything less, unless you are on top of the water perameters. I have one goldfish in a five foot tank. Theoretically could have 3 maybe, definitely 2 - but they really do need a large volume of water to stand a chance.

As above plus get some air bubbles in there pronto.

The 'significant lump' could be a tumour, of course, in which case euthenasia sadly could be the way to go, as per Camer.

If it isn't a tumour, it could be reaction to internal parasites and/or an internal bacterial infection. There are off-the-shelf treatments which are usually effective (though less so the more advanced the problem is).

A further possibility is that it has a gut blockage - which places pressure on the swimbladder just as do the conditions mentioned above. However, if it is a blockage, there's at least something relatively gentle that you can try :

If the fish is eating, insert a single crystal of Epsom Salts into half a pea and make sure that the poorly fish eats it. If the fish isn't eating, then putting it briefly into a bath of tankwater (as per Hogzilla) that has a small amount of Epsom Salts dissolved in it may help.

Has its diet been good in general - lots of veggies etc.?

are they fancy goldfish or common/comet goldfish? fancies are more prone to bouyancy issues, especially if fed floating type diets with no greens regularly. feeding cooked shelled peas at least a couple of times per week is a must for all goldfish IMO, but especially fancy goldfish. i also only feed sinking foods, i stick with saki hikari sinking mini pellets.

if it is floating with any portion of its body exposed to the air it will suffer with its skin drying out, and you really need to euthanise.

if you do decide to euthanise, do not flush the fish. either blunt trauma to the head, decapitate, or my preferred method is clove oil mixed into a small amount of tank water until a milky colour, added to a bowl with the fish and enough water to cover its body. it will put the fish to sleep, and then add more clove oil to stop the heart. its the kindest way IMO, very little stress, and if done a little at a time until it floats on its side or upside down then theres no reaction to the clove oil being added to the water. if you add too much at once they can stress a little but they quickly fall asleep either way.

10gallons per fish is for fancy type goldfish, fat bodies, twin tails. this is also 10g per fish after 20g for the 1st fish. so a pair would need 30g. for your 6 if fancy they would need a 70g tank, or at least double that if common streamlined single tailed types.

7 years isnt nearly half the life expectancy of a goldfish so hopefully it wont be something that cant be fixed.

I didn't know that! when i get my next GF's I shall try it out with them.

(Last GF died last year at 13. Been meaning to get a couple more but haven't got round to it, although the tank is cleaned and ready to be filled and cycled)