I had a problem in my CRS tank with the MTS would all head toward the surface, especially in the morning. They would even all line up at the top or a few inches from the top. My CRS would also hitch a ride on the snails and hang out at the surface. I realized I had a film on the surface of the water and the outflow of the cannister wasn't breaking it up. I moved the waterflow up and broke up the film. It worked, the snails and shrimp stopped heading toward the surface. I can only assume the film was blocking the gas exchange and the O2 was to low.