While Phil's advice matches what the extension books say, it's what they don't say that people need to understand. The books don't tell you there will literally be thousands of small tilapia taking up space and that the catfish won't eat the little darlings, nor will those baby tilapia grow big enough to feed you. You'll have a bunch of small fish to do something with, like make cat food. Books also won't tell you that tilapia can't withstand high levels of ammonia or its byproducts as it de-nitrifies in your water...and that tilapia are notorious for producing lots of ammonia via metabolism. Those papers also don't tell you that different types of water chemistry are affected by biological byproducts from fish metabolism. I've raised channel catfish and tilapia in combination. Catfish growth rates don't seem to be affected differently by the presence of tilapia as long as we properly fed them. Both species will actively chase the feed. Catfish have a slight competitive advantage because they are bigger. With both species, each time I did it, was disappointed at the end of the year to have so few tilapia big enough to eat and so many small ones that didn't have a place in our program in November. We'd bring the 4" fish into the aquaponics system I had back then, four 1,000 gallon tanks, and overwinter them to sell the next year, but the majority of poundage of tilapia were too small to eat. We did that about four years in a row and gave up. We'd only end up with about 15-20 pounds of tilapia fillets (ever filleted a 6" tilapia?) and 60-70 pounds of catfish carcasses for the freezer. I could tell when the water quality was about to go south when the bloom changed and fish became slightly lethargic with feeding, so I'd drain off a foot of water and then add fresh well water. That became a bi-weekly practice by August, until we drained each pond in November. In my opinion, from doing that combination long enough to see how it really worked, the juice wasn't worth the squeeze for the return on investment to feed our family. Shifted those hatchery ponds to bluegills to sell for stocking other's ponds. That juice was worth it.
Phil is correct with his reference to the literature, but like Paul Harvey said during his decades on the radio waves, "And that's the Rest of the Story." Sometimes the crossover between the literature and reality has some gaps. Ask me how I know.😊
