Yeah that's why i generally avoid community tanks all of my tanks are biotope meaning all the fish are from the same region i mean humans are 87268763909873089763 times smarter than fish and look how long it took us to accept people from other nations lol... but seriously i'd stay away from anything anti bacterial it could wipe out all your nitrogen cycle bacteria. also it sounds like your changing your water alot i mean it's not bad but it's unnessisary work... also anyone who tells you they have 0 nitrate in their tank is lying or has a faulty testing kit the only thing that gets rid or nitrate is plants i mean water changes dilute it but never totally get rid of it. Well some of it escapes as nitrogen gas but not as much as i like to fantasize haha. Your pleco should be fine you can always try adding some target fish like danio's or tetra's especially tetra's if your diggin my biotope idea considering the bristle nose is an amazonian so are the angels. But he'll eventually learn how to hide better and grow. Plus fish are fickle they'll loose interest in him eventually. Sometimes people get too picky about their fish's behavior think about how much fin nipping goes on in the wild i tend not to worry about unless i see some kind of infection developing then it's time for quarentine and treatment. One more random out there idea..try changing the temp a couple of degrees temperature changes can spur on mating and other ritualist seasonal behaviors stands to reason maybe it would change the mood of your tank. (just a random thought) PS dont forget algea waffers at night when it's totally dark that will allow him to eat with out harassment from the other day dwelling fish.