Guys the filter is an Elite Stingray 15 and the instructions do not mention placing the top above the water line. However I've now adjusted the filter so the top is slightly touching the surface and the difference in bubble production is huge, so I'm now assuming that the tanks O2 production is...
I recently purchased an all in one aquarium set which apparently has everything I need to get going, apart from the fish of course. It included a filter which when I put in the water does produce the odd bubble. However when I was a kid I remember the family aquarium had a proper pump with a top...