I agree with Demeter. Add a sponge filter attachment to the air stone.
You are likely dealing with a fish-in cycle process. My guess is that there is a build up of ammonia in the tank, and even weekly 50% water changes weekly just won't fix that. The ammonia should be zero constantly, and the sponge filter will be a great home for the beneficial bacteria we need to help with that.
The floating plants could be helping a lot with that, and depending on how many fish you have in there and how much floating plants, they might be dealing with the ammonia sufficiently to keep the fish alive.
But... I'd suggest that, even with that, the sponge filter will make for an even better environment. (Just squeeze out the gunk caught by the filter with each water change, in dechlorinated water - even old tank water will work great - and things will be far more healthy in the tub.)