that aquarium doesn't appear to have enough land area for him. I would use a sponge filter because he is less likely to be caught in one, if one at all. you clean the water very frequently so I don't see a need to use a filter. I never use a filter and I don't clean water as often, and everything is fine. he can quickly drown if trapped in a regular filter. additionally, species mixing isn't a good idea. if the toad swallows a fish, the fins/bones of the fish could clog him and kill him, and the secretions produced by the other frogs may hurt him. he may also not like freeze dried foods. try pieces of earthworm (still moving if possible) or live crickets/roaches. he doesn't look too thin so he should be fine. these guys can go weeks without food. another thing that can be a problem is that harsh light over the whole tank. I would use a smaller bulb and be sure the water temp is between 62 and 66 degrees for the firebelly