That's fine. Plecos can handle a pretty wide range- they are not a super warm lover so I wouldn't venture any higher than 78-80. Get your ammonia lowered, this could be negatively effecting your fish.
If your pleco is eating meaty foods, it could be taking on color because of that.
Platys, Tetras, Mollies, Bettas, BN Plecos
Red Eared Slider