Take off the venturi (air line sticking out of the powerhead) and you won't have the bubbles in the tank. Actually, the way you have it set up, it's suposed to put a lot of bublbles in the tank...
Don't have any ideas to reduce the flow, other than the fact that undergravel filters can be a major detriment if not maintained well and it apears that you have a power filter on the tank, so it shouldn't really be nessisary depending on the size of the filter.