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Floating fish

Subject: Floating fish
by JerryTheFish on 13/5/2019 21:14:59

My fish named Jerry (a goldfish koi hybrid of some sort as all the fish smart people I’ve asked have told me) has been acting very strange the last few days. He keeps sitting at the top of the water in the bottom right hand corner of his tank. It’s only this corner he seems to be gravitating to, he won’t sit in the other corners of the tank just this one.
Normally I would think it would be lack of oxygen or poor quality water as he is sitting opposite the filter, but he hasn’t got laboured breathing, not gasping for air at all and is only sitting there sometimes. He can be swimming around all day normally, then may be sitting there most of the evening. It’s very on and off him doing this the last few days, so it’s not all the time.
He is in a 54L tank I think it is, has been set up since October. no other fish, no new ornaments or plants. It’s temperature is between 19 and 21 as it has always been. I’ve tested his ph, ammonia, nitrite and nitrate, all the same as they have always been.
He is eating happily, swimming around normally, no laboured breathing/gulping for air, he is literally just swimming.
I did a 40%(ish) water change yesterday, but it hasn’t made a difference. Any ideas? I love my little guy! Thanks in advance!