recently my rainbowfish have been spazzing out and eventually dying. they will start to fly around the tank then stop like they are paralyzed and sink to the bottom. they eventually come too and start swimming but the process repeats itself and the end up dying. I tested my water and everything is normal so i decided to do about a half water change then let it filter over night and everything seemed fine but i have it happing again to another fish. the others seem to be doing ok thank god but i was wondering if anybody had any idea as to what was/is going on?

Thanks