Q: MM wrote,

You’ve helped me in the past with one of my Bettas (he had swim bladder trouble). I have a quick question about one of my goldfish. I have 2 large goldfish (almost about a foot long, each), also one large algae eater and a small bottom feeder (15 yrs. old) lives in this cave and hardly ever comes out. They’re all in a 50-gallon with filters, etc. For the last month, one of my large goldfish has been acting very strangely, but only when I turn the aquarium lights out. He starts swimming upside down, and sort of “spinning and twisting”. It’s like he has no idea where the top and bottom of the tank are once I turn the lights off. I’ve tried leaving one light on for him in the tank, but then the fish swim around all night and don’t sink to the bottom and sleep like usual…all because of the light being on, they get confused and don’t know when it’s daytime/night. We hear him splashing around all night and I can’t figure out why he won’t sleep like usual alongside the other goldfish (that’s what they would always do). He just keeps swimming upside down and doing this twisting motion all the way to the top. As soon as it starts getting light outside again, he’s perfectly fine (even before I turn the aquarium lights on). I’ve even tried putting a night-light next to the aquarium, but that doesn’t help. He only swims normally when both aquarium lights are on or when it’s daytime (early a.m. before I turn their lights on). Now sometimes he does look a little topsy-turvy even during the day, but it’s NOTHING compared to how he swims at night…it’s just awful to watch him struggling like that. This other fish-guy told me that maybe the algae eater is chasing him at night and he’s having some kind of nervous reaction to it…but that’s not it. I’ve watched them and that algae eater has no interest in anything besides his algae wafers :-) Any advice would be greatly appreciated…I’m thinking maybe it’s his eyesight? Thanks again.

This is pretty strange behavior. I understand why the fish guy blamed the algae eater as many have a bad habit of attaching themselves to goldfish at night and sucking on their slime coats. If you don’t think this is it, then I trust your judgment. You’d probably catch him in the act anyway.

That said it could be some sort of visual impairment like cataracts or cloudy-eye but in both of those instances you can see the filmy coating on the fish’s eye. If your goldfish’s eyeballs look clear then I might consider another reason for his anxiety. It’s possible they are out-growing their aquarium and he’s feeling cramped or that he is seeing some sort of reflection when the lights are off. (Though usually reflections are caused when the lights are on.) Swim bladder disorder is also very common among goldfish so you could be experiencing the same problem as you did with your Betta.

I’m afraid I have no experience with goldfish so I can only give general suggestions of what the problem might be. I do recommend checking all the water parameters to make sure there aren’t any toxins in the water and that the ph, gh and temperature are all at good levels. I also recommend chatting with some folks who know their cold water fish as some of the illnesses for them may be different then with tropicals which I’m more familiar with.

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