Pygmy corys not looking well.

[xander]

I have four pygmy corys in a 30 gallon tank housed with 3 rubbernose plecos and 7 green fire tetras. I have eco complete as the substrate with some dark driftwood and a piece of slate. The tank has been set up for almost a month. I skipped starting a cycle by having the intended filter (eheim ecco 2234) run on my 125 for a few months before starting this 30 gallon. I started it with a few baby swords and what I'm guessing might be "star" something (fast growing hygro sort of plant). I put the rubbernose which I got at a LFS and the pygmy corys from another 30 gallon tank in on the day I set it up. In the previous tank they always hid from view. In this tank as soon as they settled they were all over the tank. Up and down and all around chasing each other around. It seemed like they were very happy in this setup. Well about a week ago I did a 20% PWC and installed a sponge prefilter onto the intake of the ecco 2234. After that they corys seemed to have trouble swimming where they want to. They will swim in spirals and when at rest tend to be on their backs. I thought it was odd that all 4 would do this. I at first assumed some sort of swim bladder problem since they seem fine other than the way they get around. No wounds or missing appendages. I recently did another PWC and put a few peas in there for them to eat. There is no improvement on their motor skills. The other fish seem fine. The green fire tetras I put in from my 125 are happier than ever in this 30 and the males are showing breeding colors. My KH is 4, the GH is 7, nitrates are less than 10, no ammonia or nitrites. The pH was hard to read but I am thinking its around 7 if not its at least stable. I don't know what to do. My water quality is good and the corys are still eating but I'm worried. It can't be good to just be laying on their backs and running into stuff when they try to get somewhere. Any suggestions?