My toads sit in the water most of the time. The are not swimming all the time but they do sit on rocks that are submerged a little bit in the water. Maybe if I took away the rock option and they had to just float they wouldn't spend so much time in the water and fish would be okay. Or maybe with your paludarium your plants are helping to filter the toxins....hmm. How long have you had the toad in there with the fish and what kind a fish are you keeping? Like I said I had five gallons of water in my tank but I didn't have any live plants and the guppies croaked quickly with just one toad introduced. How many gallons of water do you have in your tank and are you using a large biological filter? Maybe with more water and a lager filter that would help.