Can't keep pleco's alive

[rudie]

I have got a catfishtank for about 7 month's now with the following stock and parameters.

1.8m x 65 cm x 65 cm
ph = 7.6
temp = 26 Degrees Celcius
ammonia = 0ppm
nitrite = 5 ppm

Stock
2 x S.Eupterus
2 x S.Petricola
1 x Pictus Cat
2 x Spotted talking catfish
4 x A.dolichopterus
2 x A.(not sure)
2 x chinese algea eaters
2 x C.Fowleri
2 x C.Arcuatus
3 x C.Aeneus
1 x C.Sterbai
2 x C.Napoensis
2 x C.Adolfoi
1 x Whiptail
1 x Red tail shark
5 x tiger barbs
5 x tiger barbs albino

The last 3 entries were purchased for some midwater swimming.

Now here is the thing, I purchased a Hi-fin leopard plec, the guy was in there for about 2 months when one day he just died, I then purchased a albino plec, this time he lasted 3 months and then also just died. Now both of them was eating very well and looked in tip top condition. Could my PH be wrong for them as I love these plecs and would love to keep them.

[Barbie]

Nitrites are toxic. In an established system with a "cycled" filter you should have a 0 reading. They affect the bloods ability to carry oxygen. What filtration do you have on the tank? How often are you doing water changes and how much when you do them? The pH should not be your problem, unless you're attempting to modify it while in the tank and causing swings. It sounds like it's definitely a busy tank!

Barbie