Problem with Syn.Petricola's

[Bonnie]

I have 4 S.Petricolas, they've been in the tank from the beginning, 2 years now. Always looked good, they hide alot and swim around occasionally,ok. I know wild SP's would normally have a tank with high ph. The breeder I bought them from bred them in a neutral ph 7.0, and that's what my tank was at the beginning. I recently started noticing them coming up to the top of the tank and actually bobbing their heads out of the water, and 2 of them look extremely thin, in fact they look like big heads with skinny bodies! None of the other fish are having any problems. All params are fine, 0Ammonia,0 nitrites and 0-5ppm nitrates, the only thing is over time the ph went all the way down to 6.0. I thought maybe they weren't getting enough food so I would put food at night in their area of the tank, in fact I'm probably putting too much food in the tank. Could it be parasites? But if it was then other fish would be having problems as well. I usually clean the tanks once a week, however summer is a busy time so it's every 2 weeks, I went back to once a week. Could it be possible I have too many bottom dwellers? 2 super red BN's, and one whiptail cat, the tank is 75g's. other then the ph I can't figure out what is happening to them. If anyone has some experience with them, if you have any ideas please let me know, I really like these guys, thanks.