Sounds like fungus gnats. DE sprinkled on the roots in the clay balls/hydroton might work. I don’t see any harm as long as you are not running misters or something else.
H2O2 will help. Part of the likely problem is low oxygen and rotting roots allowing the fungus gnats to have a viable place to lay their eggs and have the larva survive in fungus rich decomposing roots area above the water level.
Other thoughts, water level too high, temperatures too high, these might be the cause or a contributor to creating an environment that fungus gnats can survive in a hydro environment, usually you won’t really get them in hydroton hydro set-ups.