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?