Slight shimmer - I had to drop the inlet pipe further down into the tank due to evaporation (now my top is open to the elements, just lifted pipe out of tank so returning water is disturbing the water surface - I did wonder whether is was due to the heat, as It is very hot here.
This sounds ike you have got to grips with the problem.
It will still take some time for the O2 to rise, I'd guess at a few hours. If the fish are still gasping now then a WC will be much appreciated by them! I know I am repeating myself but I think it is important so your fish do not suffer needlessly.
The amazing thing is that the inlet pipe has been like that for 3 week and its strange how this issue has only just popped up, i think high ambient temperature is partially to blame as well my front room as a temp of 78 with no sun. The front room never gets any sun but the heat transfers from the back room to the front, sun is always on the back until about 4.
N.B Now with the pipe out of the water (like a water fall) make me want to go the loo all the time - you know what the sound of running water like.