Why is my cory swimming against the glass?

I have three cories in a 30 gallon tank. The only other fish in the tank right now are neons and otos, and the stocking density is light. Two of the cories act perfectly normal. One originally did, but within the last week or so has taken to swimming frantically up and down the glass. I've never seen a fish swim so frantically in my life. I've tested practically every water parameter known to man (Ammonia, nitrite, nitrate, phosphorus, iron, CO2, pH, gH, kH...), and they are all perfectly fine. The tank is heavily planted with live plans and DIY CO2, but the CO2 is supposedly in the low range (lower than it should be for the plants) and all the other fish are acting perfectly normal. The other cories are as laid back as can be, and I can see nothing physically wrong with this fish. Occassionally he calms down and stops pacing, but he seems to spend the majority of his time doing this. Any idea what's going on?

2007-01-10T14:14:01Z

I feed him and he eats with the others. It's one of the few times that he doesn't pace around. Do you think they're not letting him get enough food? They don't seem very competitive at all.

Shawn W2007-01-10T19:56:04Z

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Interestingly enough fish can not tell their reflection from another fish, my corys tend to try to get into the background thinking its open water, also if you notice yoru corys have two "nostrils" and will dart up to the top of the water and suspend their heads above the water, why? because tehy can breath air using a laberinth organ like guoramis or bettas.

lilgman4242007-01-10T15:03:44Z

I'm serious man all fish do this. I have a 30 gallon tank lke you do and My tiger barbs don't do anything BUT this. The whole day they follow their reflection on the glass. From the inside of a tank theres alot of reflection.

Anonymous2016-05-23T10:28:13Z

I'd say he's got an infection and you better get some medicine quick. A good tropical fish store could tell you which virus he has and what medication to put in the tank.

Flames Fan2007-01-10T14:35:28Z

all fish do this, it's just a game they play i guess. My gold barbs, my betta, and my ram cichlid all do this, but my barbs go real fast to the top of the water, lift a bit of water from the surface and shoot back down, it's not anything to be worried about. This fish is probably just more active than the others.

Chafed2007-01-10T21:00:10Z

hehe, It's just the way that they swim/school when they are happy. I have a shoal of 3 also and their swimming behavior is what makes me love them so much.

I'm trying to order more, julii corys are pretty rare where I live.
They will swim a ton more like that when they have 6 or more in their shoal (group). Its just the way they travel. They will often replicate exactly their shoal mates' movements. =D

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