Hi! I need some help. I have a white tetra, also two black tetras in a sixty five gallon tank. Tank had been set up for years, I have had the white one fir about four years and the other two about two years. They are the only fish in the tank. The white one started laying on bottom bottom about three days ago. He will still swim around and eats. After a short while, he drifts back to his little corner, alternating between laying on his stomach and side. He's not breathing funny, just looks like he's resting. The first day I went ahead and did a fifty percent water change, vacuumed the gravel. Next day I did a test on water. The PH is perfect, no ammonia, no nitrates. I'm baffled. I cannot find any answers. He doesn't have Any injuries or physical issues. I've had him a while and if there is anything I can do, I'll do it. The test I use is a Doc Wellfish, using the test tubes and drops. I've tested it about six times on Last three days. Just tested again, all is perfect.