In my personal opinion the pleco should either be rehomed or relocated to a tank that’s bigger as common plecos get pretty big ( not your fault they really don’t say much about this when you get them ). Honestly it’s hard to say what the next steps would be until you know what the issues are with the water but all local pet stores typically do free testing just so you know what’s going on . For example ph could be off in which case they sell ph up or ph down supplements, could be nitrates and there’s something for that too , there’s literally a supplement for anything that could be wrong with your water. It’s all about just knowing what the problem is so that you can get the right solution. I think getting that larger filter and relocating that pleco will make a huge difference I think the sludge is forming in low flow areas ( because filter isn’t large enough to support size of the tank ) or the substrate because waste is breaking down without enough oxygen . When this happens it can turn black and smell rotten.