What kind of pleco did you have in with the discus? I strongly recommend BN plecos (Ancistrus sp.) for your tank. Your 180g tank could easily support 8 fully grown adults. They do a wonderful job with keeping algae off of the glass and off of plants. They don't eat plants either. It isn't common for BN plecos to go after discus. There are some other BN plecos that used to be common in the hobby before the Ancistrus sp. that were known to go after discus, but the Ancistrus don't go after them. Of course there is always the exception to the rule, but I have 3 that are 3" in my 55g with a group of sub-adults and about 6 of various sizes, including a MASSIVE 4.5+" female in my 100g with discus and have never had any problems whatsoever.