Filtration is a Cetus, with 100 micron screen, then the water goes through a 20" 50micron canister filter. These get washed on a daily basis. I fed them twice sometimes three times per day. Hard to say how many pounds of food I'd go through. I do have a question though, on the sides of the pond the plastic has a brown coating on it, algae like. Is this beneficial bacteria, or do I want to scrub it off?
I'm only seeing mechanical filtration here. No biological except for the tank surfaces. This in itself would explain your losses and they will only get worse.
I'm guessing ammona is quite high for tilapa to die. You're lucky you don't have bluegill or yellow perch in the tank. They would all be dead.
What is your Ph? If your Ph is high it will compound your unionized ammonia which is the component of ammonia that is the toxic one.
Sorry for being blunt but just telling you like it is. I have three hundred tilapia and bluegill up to 6 inches in a 600 gallon tank with no issues. However I have 4 cubic feet of plastic media for biofiltration along with a mechanical filter. The water also runs through a 48 cubic foot raft tank for additional biofiltration and I keep the ph slightly acidic.
