Hello! I have what I believe to be a common pleco in a large tank with one goldfish. I got him to replace my last pleco who died as well but I had him for many years. Anyways I have had "Big Bob" for about 3 months now. We bought him when he was already quite large. He has been healthy and happy the past 3 months and this morning I noticed his eyes were sunken in. I feed him algae wafers and he eats the goldfish food as well...he is not in his usual hiding spit during the day he is lying right out in the rocks and looking lethargic. I offered him a wafer and cucumber but he is not interested. He tried to suck on the glass and kind of fell down like he couldn't get a good grip. Tested water which is freshwater with air bubbler and 24/7 filter, partial water changes done monthly using cycle, Aquarian salt and aqua plus. test came back as such: GH- 180. KH - lightest colour..0? pH - 6.0/6.5, nitrites - 0.5, and nitrates VERY dark 200. Very bad!! I am cycling some water for a water change as we speak. Goldfish is doing just fine. Added some more cycle as well...not sure what else to do at this point any advice is appreciated.