I didn't like the look of my airstone in my aquariums and the fact that my fish would keep digging them up, so I moved them into the back of my HOB filters. Is there anything wrong with that? Or is it perfectly fine
its fine as long as your filter is moving the top water bublers are mainly for looks and if you have problems them digging it up there are airstones with suction cups at the bottom so they dont move and stay put you can pick them up at any fish store
Having a HOB style filter you already have plenty of surface agitation. Unless you were at 10 times the stock level you should be there most likely is no Co2 build up in your tank.
Very very very little oxygen is actually added to the water by the bubbles themselves... It's the surface agitation plus circulation that oxygenates the water...
And your HOB alone will likely agitate your water plenty...
Putting the bubbler in the HOB will not add oxygen to the water. The water in the HOB is already agitated at the surface and well circulated.