I have kept fish and cats for many years and never had a problem with them. My tanks are large and have metal lids. As winter comes on again Borris is using the tank in the living room more and more often as a heat pad during the day when there is nobody home (he lies across the lid as the lights keep it lovely and warm). He enjoys eyeballing the fish ( a large Marbled Oscar in this tank) and the fish has just as much fun eyeballing him back! As long as lids are strong and secure you should be fine. If the cat learns to lift the lid a couple of elastic bands should secure it. House plants on the top will love the humidity and warmth and keep the lid shut too but be careful not to block any ventilation holes.