When our fish excrete waste, it contains a very high percentage of ammonia. When the water filters through the grow bed, bacteria called nitrosomanas bacteria convert the ammonia into nitrites. Nitrite is another harmful toxin that is nearly as dangerous to the fish as ammonia and a second group of bacteria known as nitrobacter are required to convert the nitrite into nitrate. Nitrates are far less harmfull to your fish but once again still toxic above a certain level. Fortunately for us most plants require nitrates to grow and flourish and they will complete the cycle of removing the ammonia from your tank.
A healthy, well balanced and established aquaponics system will have less than 0.25 ppm of either ammonia, nitrite or nitrate.
The ideal growbed environment is at least 1 foot deep. If your growbed is shallower than this the bacteria will still colonise the growbed but conditions will be less than ideal for them and the aquaponics system may not perform as well as it would otherwise. They need oxygen in the water to live and work.
As a general rule a brand new system will require about 4 weeks to cycle at around 20˚C. It will take longer in colder water. The optimal temperature is 25-30 C.
Many people with aquaponics systems try to maintain their pH at around 7.0 to 7.2 because this range satisfies the plants, fish and bacteria. The nitrogen cycle itself has a tendency to reduce pH, however
it is pretty easy to keep pH at around 7.4 through the addition of calcium carbonate. Calcium carbonate increases pH, but will stop dissolving at pH around 7.4, meaning pH will stay pretty stable until all of the available calcium carbonate is depleted.