How to properly cycle your pond or system for tilapia
What is "cycling"?
Cycling is the word that fish enthusiasts use to describe the process of "establishing" the nitrifying bacteria colonies in tanks, aquariums and ponds. Many fish are very sensitive to even low levels of un-ionized ammonia and nitrite. So before an aquarist dares to put their $200 ornamental into a newly set up system, it would be wise for them to make sure that the Nitrosomonas and Nitrobacter were well established.
The "cycle" refers to the specific pattern that is visible on test kits and equipment. First, the total ammonia tests at 0 ppm, then slowly rises to 8 ppm and beyond. After the ammonia has been rising for a few days, nitrite appears, and rises from 0 ppm to 5 ppm and higher. As the nitrite rises, the ammonia starts to drop back down to 0 ppm. Soon after nitrite starts to rise, nitrate appears, and starts its rise. The rise in nitrate also signals the eventual fall of nitrite. Eventually, ammonia and nitrite drop to 0 ppm, and nitrate starts a slow climb upward. At this point the "cycle" is complete.
Normally, people use less expensive fish to "pre-cycle" their aquariums. For freshwater, most people use goldfish, and in saltwater, most people use damsels. These are normally characterized as "sacrificial" fish by pet stores, hoping to sell the more profitable cichlids and tangs after the nitrifying bacteria is established.
Is pre-cycling necessary before introducing tilapia to your system?
Absolutely not! In fact, it's a complete waste of time.
Most of the knowledge that people have about food fish comes from raising ornamental specimens at home in their aquarium. However, this knowledge is not applicable to raising tilapia for farming or for aquaponics purposes. Of course, this does not prevent well-intended people from making authoritative statements about cycling aquaculture or aquaponics systems. The world is full of armchair fish experts who tend to overreach, without actually knowing.
Of course, it would be easy for me to just make the above statements and walk away, but doing so would put me in the same category as all of the other "experts". So I'll explain how to properly cycle your pond or system for tilapia.
Why pre-cycling isn't required in tilapia farming and aquaponics.
Pre-cycling isn't required because the tilapia aren't in any danger during the process. The first toxin of the nitrification cycle is ammonia, but at a pH of 8.0 or less, ammonia is almost completely non-toxic to tilapia. In aquaponics, where pH is typically kept below 7.0, the toxic form of ammonia barely even exists. The next toxic compound is nitrite, which is actually toxic to tilapia. However, the only affect that nitrite has on tilapia, is to reduce the amount of oxygen that their blood can absorb. At the levels found during the nitrification cycle, it isn't deadly to tilapia. Unless of course, you force them to exert themselves. So do your best not to scare them for a few days while the Nitrobacter oxidizes the nitrite. The last part of the cycle, nitrate, simply isn't toxic at the levels found in tilapia farming.
Another reason that pre-cycling is useless, has to do with the fact that the number of bacteria colonies supported is ultimately determined by the available ammonia. Goldfish put out very little ammonia compared to tilapia. So a system that has established nitrosomonas and nitrobacter colonies, in sufficient quantities for goldfish, will instantly be overwhelmed by tilapia. The "cycle" will start again, resulting with a rise in ammonia and nitrite, as if the goldfish were never there at all.
So, in a nutshell, the best way to cycle a brand new system, is to keep the pH at 7.0 or below, and let the nitrifying bacteria adjust their numbers to accommodate the ammonia output of your tilapia.