When you have your pump and skimmer on the water should be halfway at the black return hose outlet in the back. When I do water changes once the water starts going into the back I turn the pump on and fill the tank until it reaches that level in the back if that makes sense... sounds like you might just need to add more water, if it gets low in the back it tends to kick out micro bubblesnice looking progress man! I have not altered anything in the back of mine, but it seems to get like, air pockets underneath everything in the back? Water levels all look normal, return nozzle starts putting out micro bubbles, and when i turn the pump off and turn it back on, the water lever in the back is half empty. Any experience with anything like that?