Pleco Trouble

Hi everyone -

I'd appreciate any thoughts on this. For some reason, I can't seem to keep pleco's in my water.

I had a 12 gallon eclipse system, which was way too small. I tried a couple in that tank and they all do the same thing. They would be active to start but then gradually become less active, not eat, and eventually die. I was a little ignorant at the time, I was putting small common plecos in the 12 gallon, but regardless, they didn't live.

I've since upgraded to a 55 about 4 months ago, hoping for more stable water conditions, more space, etc. All my parameters are good, the other fish I have show no signs of stress, etc. I decided to try a small bristlenose pleco in the new tank. The same things happen. It gradually becomes less active and dies. I've put bottom feeder tabs, blanched cucumber, etc to be sure there was food available to him. I even got a piece of driftwood after reading they need in their diet. I've now had this happen to 2 bristlenoses. It is my understanding that the city water here is hard, but I don't have a test kit for hardness and I don't know if that would kill them.

I'm assuming I'm just going to give up on the plecos! I have an oto in there now and he does a good job cleaning. Anyone else have similar experience. Is there something I'm missing?

I test for everything but hardness.

Ammonia is 0
Nitrite is 0
Nitrate 10-20
PH is 7.4-7.6

I have two bala sharks, four cory cats, an oto, and a couple tetras (tetra's are transferred from the small tank). I haven't lost any other fish, and they don't even seem stressed or anything.

I have a heater and keep the tank around 79F. The pleco does seem to spend a large amount of time hanging around the heater, but i'd think 79 would be a decent temperature for a community tank from what I've read.

It has me puzzled (and frustrated!). If there were more signs of trouble, it would make more sense but it only seems to be the plecos..........so I don't know!