I've been having issues with my nano tank, I had my fish die off, waited for the potential disease to pass, then restocked. Everyone was doing fine for a week then I had another couple die off. The water parameters, PH, nitrates, and nitrites have all been fine for every test. I change 1/2 of the water every 1-2 weeks. I have a ton of very healthy plants. I am starting to suspect the issue is a lack of surface agitation. I have seen a film build up on the surface and last night a couple fish were opening and closing their mouths quickly. Is there a lack of oxygen in the water? I am considering buying a new filter that has the water spout at the top or a spray bar arm that can protrude from the water slightly. Does this sound like it could fix my issue?