Don't listen to the flamers rcnjr; whilst I don't condone over crowding, I'm not a fish snob that believes fish require a good acre to stretch their legs, and if you give them any less, you're a sick sick monster. If people stop and think about a fishes swimming tendencies, i.e. Bottom dwellers, mid-tank, and finally top. If I have a 10g with 2 bns (opposing sexes) a 4-5 corys, and 6 ...hatchets the tank would be far from overcrowded - all that'd be of concern is params. Which in this case are perfectly fine. So rather than flame someone whos concerned about their tank and is looking to figure out a problem, not have your opinions pushed down his/her throat, post if you have a RELEVANT suggestion, if not, go and blab away on a thread you know something about. Doing something for a long time doesn't mean you know best.
Rcnr; how thick is your gravel? If its too thick and not being distrubed frequently when you gravel vac/do maintenance, gases can build up and kill fish really quick. Is the albino plec you've got in there fully matured? if its mature and male it may be attacking the new plecs during the night and contributing to death by stress. Did you buy the new ones from the same store? It's possible the stock may be sick and again stress of moving is simply finishing them off. Have you checked the corpses for obvious causes of deaths?