FX5 filling with air

I recently added a UV filter - hang off the side type - and placed the output near the input of my FX5. The output is a very low flow waterfall with barely a ripple on the surface. But it did make a few very small bubbles. I'm talking about the kind of bubbles that are so small they go down instead of up. But anyway, the FX5 was acting up and pulsing a lot with some fine bubbles coming out. So I shut it off. As soon as I did, a ton of air came out the output almost like when you first plug it in. I waited about 5 minutes and turned it on again, it again belched air. It primed itself up and is now air free. I've only had the UV on the tank for a few days. I can't see how that little bit of air filled the FX5 and just sat there. But anyway, I move the UV to the other side of the tank and hopefully the air problems will go away. I also added 5 gallons of water to bring the level almost to the very top to avoid any unecessary bubbles from the UV output.