I noticed tonight that my dwarf puffers were hanging out near the top of the tank. It seems like the otos are also. Could this be a sign of oxygen deprivation? I did a PWC just in case but I'm wondering if I should add an airstone. It's a 5gal planted with DIY CO2 with 2 DPs, 2otos and a halfbeak. Haven't had time to test the water yet but I will tomorrow...