first,,, even one h. pleco is gonna get too big for a 10g tank. second they are
HUGE waste producers,,, so your
ammonia and nitrites are probably way too high. Do you test your water?
API master water tester is great. You need to take the healthy pleco and molly back to the store, or buy a 55g + tank for each pleco. They can get up to 18" and the rule of thumb is 1" of adult fish to 1gallon of water, but since these guys produce so much waste, triple that. that can cause
nitrite and ammonia poisoning. The health of your other fish you really need to take them back or find new homes for them.