People recommend warmer temps for treating ich because it increase the cycle of the parasite making it easier to kill (or if the heat is high enough it can out right kill some strain); however this is a short term thing. The pleco might prefer warmer temps but most species of cory are going to be happy at 70; and definitely happier than the 78 or 80 that some people insist on keeping them at. Having said this i certainly have some fishes that would be very happy at 70 or even 75 - certainly my discus begin to throw a tantrum when the temp gets below 82.5 and they get really sad at 81; and i believe the larger geo i have prefer high 70s low 80s though they are accepting of mid-upper 70s.
It all comes down to species. A lot of my dwarf cicihld will produce female heavy spawns in the mid to low 70s and if you want closer to 50:50 i need mid-upper 70s.
