As the saying oges, it never rains but it pours.
But glad to hear things are improving.
Air trapped in canisters is common, the store obviously sorted that out. A trick for when you next take it apart to rinse the media: When you put it back together, fill the canister with water almost to the top and gently shake it a bit to dislodge air pockets. Then connect the hoses and turn it on. This usually helps to get it started and there will be less trapped air. I sometimes have air for a few hours in my canisters even after a normal water change, if the water level has lowered to the filter intake which in my case it sometimes does.
On the light, I assume these are incandescent (screw-in) bulbs, and compact fluiorescent? That's fine. Six hours is sufficient, you could try 8 if you like. Is the algae brown or green?