A testing kit is an absolute necessity. I would remove the other fish and lower the PH to 6.5. I have my Discus in a species tank and they have never been happier. They were originally in my community tank. mine are rather shy when you move towards the tank, but from where i sit in my living room, they move beautifully round the tank. Is the tank planted? they love a planted tank. I bought a Turqouise red from the same place and he soon took over as the alpha male, and I have never had any problems with him. I can't tell you how impressed i was by the store, their service, fish and the way that he was packaged. Since lowering the PH in the tank, it has increased their activity again. I tried using peat in the filter, trying to lower the PH naturally, but with limited success. I use Waterlife Humaquat and it go the PH spot on in 24 hrs. It does turn the water a SLIGHT tea colour, but so does peat. Feel free to message me, would love to chat about this sometime, as discus are my passion!