I have an approximately 8 inch common pleco in a 55g with 2 oscars and 3 pictus cats. Lately, the pleco has been swimming to the surface like a cory cat and releasing bubbles when he gets back to the bottom. I have had him for almost a year and have never noticed him doing anything like this. My nitrate, ammonia and nitrite are all zero. My pH is 6.5. I recently lowerred the water level about 3 inches because my oscars were smashing their heads into the hood. Could this have caused him to behave this way or is it a sign of something else? Or is it nothing at all? Also, I am aware that this size tank is not adequate for all of these fish when they are full grown. What size do you think would be the minimum? I'd like to keep them together because they all get along really well. Any help would be greatly appreciated!!!