I have purchased about 12 plecos in the last 2 months. They die within 10- 12 days. No obivious sign of injury and nothing else in the tank is dying. I have a 10gal tank with 2 tetras, 2 barbs, 1 beta (which hides all of the time) and 1 corrycat. I would like to keep 1 pleco alive. What am I doing wrong?