Would like to ask for your opinion. During the last 4-5 weeks I have started to loose 2 fish per week, none shows any visible symptoms.
I have a large group of Celebes Rainbowfish, one of them have started to show erratic swimming, doing loops and swims as it saw the devil, I have lost all Otos (all were long established) and one Melanotaenia Maccullochi and few days ago a Gold Minnow has also gone, but this one always looked a bit sad. All water parameters are perfect, 0 ammonia nitrite and phosphate, 50 nitrate KH 12 GH 18 PH 8.
Last night I had a thought that I might have parasites in there, is it feasible? if so, what medication if at all? Don't know if it matters but I also have 12 Corys. Tank is 400ltr.
Parasites are unlikely if you haven't added new fish recently, and would usually involves other symptoms such as flashing against the decor and substrate for external parasites or weight loss for internal ones.
Your water is rather too hard for Otocinclus and for M.maccullochi, and right on the upper limit for the minnows so that may explain their demise.