I have not used those Hydroton balls of clay but I am wondering if you should also remove a couple of them to be sure that the eggs are just up near the top. They insects could be laying eggs all the way from top to bottom so it might take more than just spraying the surface.
Can remember having fish tails i know they dont hurt the plant but yuk after a water. Good luck my friend hope its
not to bad but smokingwings got a good point there how far does it go down.