Like the topic says, fish are gasping for air, and my bottom feeders(pleco's and catfish) are breathing very heavy. Just started today, its seems lack of oxygen, just don't know what caused it or what to do. Ammonia and nitrate are at zero, PH is at about 6.4 which is low but that been a problem of mine with this tank for about 6 months now. My theory for the low PH is the tank is overcrowed, I have 18 fish in a 36 gallon tank. The average size of the fish is about three inches, I do weekly water changes. The water is a little cloudy so a algea bloom might be coming, would that cause the fish to gasp for air? Any idea's?
Thanks

hi
your tank is way over stocked, plus u could do with a air stone and airproducer putting oxygen in there. and real plants to consume the co2 being produced

Also if you have a hang on back filter lower your water level a little so the water splashes a little when it's being put back into the tank. Breaking the surface tention on the water's surface brings oxygen into the tank.