I need help with my african dwarf frogs !

Hi! I'm brand new to the forum and after searching the inter webs for some type of help I decided the best thing to do would be to join a forum .I had 5 African dwarf frogs in a heated / filtered 5.5 gallon they were all doing really well very active eating etc etc . Last Wednesday when I was cleaning up their tank I noticed one dead in their "food" dish. I immediately removed him and proceeded with my water change and cleaned the tank decorations plants etc. I have since upgraded and moved the remaining 4 into a filter/ heated ten gallon. I noticed that one frog in particular had a cloudy looking halo around his back half and his eyes are cloudy . I decreased tank temp from 80 to 76 . The cloudy substance that was around his body is now gone, but his eyes still look horrible , he is super active and is eating but I really don't think he can see well at all ( I know they have poor vision to begin with) . I've been researching what I can put in the tank and all I ca seem to come across is pimafix ...is this safe for an amphibian ? And should I treat the whole tank or just quarantine him in my 2.5 gallon,. HELP!