I have about 60 4" tilapia in a 270gal tank with 730 total gallons in a deep water raft system. I have noticed that about 15-20 fish hang at the top either gulping air or skimming food particles from the surface. Originally, my well water was very hard 350+, but I have recently diluted it down with sterilized river water. The water is very clear. my levels are as follows: DO: 8.0, Amomonia: .5 or less, Nitrates: 18, Nitrites<1, Hardness: 250, Alkalinity: 180, Ph: 7.4, Water Temp: 72-76 degrees F. I feed a small palmful (about 20 pellets) of adult fish food which the fish pick at until finished. There are often about 5-7 small filmy bubbles on the surface, but not excessive film. I also have about 20 2" feeder goldfish and 2 8" Koi in the same tank and these don't ever gulp air.
I exchanged 300 gallons of clean spring water today and cleaned out the crud in the solids collector, net tank, and troughs, (none which seemed excessive).
Everything I read keeps saying DO or nitrites. Both seem to be in check. I wonder about the accuracy of my DO Reader, but it is only 8 months old, and I just re-calibrated it. How often does the fluid in it need to be changed?
from my experience....when i feed the fish, afterwards there was a oil film on the water surface, the fish love to eat that oily film, so it looks like they are gulping air. second....the fish knew if someone come close to the system that there will be food, so some of them came up to the water surface. hmm...cant imagine that only 2 kois eat all food, so tilapia have nothing left, but maybe the ones who didnt catch any food are looking for some left overs. your fish look very small, maybe you should change the food, feed smaller pellets.
Gulping at the surface is usually a sign of not enough dissolved oxygen but as you say, they are not all doing it and your readings say it is high. But How much supplemental aeration do you have going? Does their behavior change if you add an air stone into the fish tank?
I don't know much about DO meters but a DO level of 8.0 in that temperature of water without lots of supplemental aeration or oxygen injection in that size tank with that many fish seems unlikely seeing as oxygen saturation in that temperature would be between 8.7-8.5 (saturation is the Max amount of oxygen the water can hold at that temperature under any natural conditions.) Anything biological going on in the system is going to use up some of that DO so unless you are driving excess aeration into every point of the system I would be shocked to think the fish are using less than a point of that dissolved oxygen.
That said, I wouldn't expect tilapia to be gulping at the surface for oxygen till you get down well below 5 ppm of DO and the fish will usually go off the feed before they start gulping for air.
Maybe you crush up some of the pellets to make them easier for the smaller tilapia to eat.