Cycling begins when ammonia is added to your system, either by you or by your fish. Ammonia (NH3) is a chemical that comes from the fish’s waste that is toxic to fish and will kill them unless it gets converted to a less toxic form of nitrogen. Unfortunately, plants can’t take up ammonia directly, so it needs to be converted to a different form of nitrogen in order to get used by plants.
However, ammonia attracts Nitrosomonas, a nitrifying bacteria that converts ammonia to nitrites (NO2). Nitrites are even more toxic to fish than ammonia, but nitrites attract another kind of nitrifying bacteria called Nitrospira, which converts nitrites to nitrates (NO3), which are non-toxic to fish and which are the best form of nitrogen for plants to take up.
Once you detect nitrates in your system and the ammonia and nitrite concentrations are below 0.5 ppm, the fishless cycling will be complete and you will be ready to add fish to your system.
In order to monitor the levels of ammonia, nitrites, and nitrates in your system, you’ll need a way to measure these chemicals. To test the levels of these different chemicals, most aquaponic gardeners use the API Freshwater Master Test Kit by Aquarium Pharmaceuticals Inc. It is easy to use and inexpensive. You will also need a submersible thermometer to measure the water temperature. The water temperature affects both the cycling rate and the health of the fish and plants once the system is cycled and the fish have been added. You may also want a dissolved oxygen (DO) test kit or meter. Dissolved oxygen will speed up the cycling process because it attracts nitrifying bacteria. Also, it is important to the health of both the fish and the plants. A DO test kit is probably not a necessity if you have enough oxygen pumping to the system and the fish don’t appear to be gasping for air, but it is helpful to have, especially if you’re planning on setting up a large system with lots of fish.
A few advantages that fishless cycling has over cycling with fish are as follows: First, it’s less stressful for both you and the fish because you don’t need to worry about keeping any fish alive during the cycling. Therefore, you don’t need to be concerned about maintaining the proper pH for the fish until you add the fish to the system. Second, when doing fishless cycling, you can raise the ammonia concentrations to a level much higher than what’s safe for fish, in order to enable the cycle to complete more quickly. When you do fishless cycling, you can finish cycling within ten days to three weeks, as opposed to four to six weeks when you cycle with fish. You also end up with a healthier bacteria base at the end of the cycling as well. Finally, you can control how much ammonia gets added to your system. If the ammonia level is getting high and there aren’t any nitrites in the system yet, you can stop adding ammonia for a few days to give the bacteria a chance to catch up. With fish, you can’t do this, of course.
One way to add ammonia to your system is by adding liquid ammonia to your system. Only use the pure form made solely of ammonia (5-10 percent by weight) and water. Don’t use anything with perfumes, soaps, colorants, surfactants, or any other additives. If the liquid foams after you shake the bottle, don’t buy it. Advantages of using liquid ammonia are that it’s relatively inexpensive, and whatever you have left over after cycling the fish, you can reuse to wash your windows. Disadvantages are that it can be hard to find if you don’t live near a cleaning supply store or hardware store.
Another way to add ammonia to the system is by using ammonium chloride. You can find it at aquarium supply stores, photography supply stores, soap supply stores, and chemical houses. Advantages are that it’s inexpensive to ship, since it’s concentrated and in dry form. A disadvantage is that it’s more expensive than liquid ammonia.
We do recommend "fishless cycling" as the way to go to set up your aquaponics habitat as humanely and balanced as possible. Once you do this prior to adding fish, your fish will be situated in a very favorable setting for their nurturing and growth. Checking the chemistry of the water is something we've learned to do, and we do it frequently, so we can always make adjustments to our system to benefit the growth of both our plants and fish. Like other types of agriculture, having a healthy ecosystem to help everything you grow thrive is key to sustainable farming!
References: Aquaponic Gardening by Sylvia Bernstein