HELP!!! Dying Fish! PH problems?

[csdax]

I've just had 2 deaths in my 55 gal, and I don't know what to do!

Last night, I found a male guppy dead, and tonight I've lost one of my young guppies (about 6 weeks old). I checked my water parameters, and the only thing unusual is that the PH is high (7.6). I've done three 20% water changes in the last 24 hours, and have been using "PH Down", but it's not changed! I don't know what the PH usually is, because I've only recently been testing it.

I don't usually have problems, and these are the first fish I've lost in about 4 months! I've had this tank for about 3 months, and have never lost any from here, even though I always use fishy cycling (plus stealing filter media from my established tanks).

My stock is:
3 swordtail (2F, 1M)
6 small adult guppies (4F, 2M) (actually, there's only 1 F adult at the moment - three are in separate breeding tanks)
4 young guppies
1 small angelfish
4 white clouds
12 assorted tetra
2 plecostomus
2 aglae eater
Tank set-up:
2 penguin 330s
200W heater
Bubble wall
Moderately planted (I added my latest 2 plants about a week ago)

:help: Any suggestions, please? I don't want to have to drop any more of my favourite fishies into my piranha tank!

[TheMightyQueenPixie]

I would chalk up those deaths to just plain old "death"...Happens...

STOP using pH down. Your water most likely has a high kh and it will keep your pH up there. At best the pH down will cause a "bounce", way worse then a constant pH of any level. I will venture a guess and say your pH naturally sits around 7.6-7.8, pretty typical for S.W. Ontario and nothing to be concerned about.

What kind of Pleco do you have in there?

[Pufferpoison]

i have to agree with queen pixie, my ph is 7.8 and have very healthy fish in 6 tanks with that ph. Fish (most) will adapt to the ph, but will not adapt to the constant ph swings. HTH

[csdax]

Thanks for the reassurance. I'll hope that the 2 deaths are just bad luck and coincidence, and keep a close eye on the rest of them.

I'm not sure what type of plecos I have - just the common-or-garden variety that I bought as a real newbie, on the advice of my local Wal-Mart (before I knew how big they can get!) They're about 2" in length, now.