Hi, I just want to know what stocking would be like in a 3ft by 3ft by 12 inches tank (W by L by H), if they were all plecs? (and maybe cories somewhere)
What sort of plecs are you interested in ?
Personally i wouldn't put any plecs that grow over 8inches in it, it would be a lot of gallons but unfortunately a 3ft long tank is not really enough length for any plecs bigger than that to go in it.
Also there is a problem with many community plec tanks as many plecs are slightly terotorial or very terrotorial towards each other, so although you could technically fit many plecs in there, there are few species of pleco's that do well together in large groups of different species.
Corys and plecs can make good community fish together, but one of the drawbacks is that many plecs produce a lot of poop and cories don't like messy substrate, so if you had many plecs you would have to be cleaning the substrate a lot more than what you would normally do.