Also, here is a bit on the nitrogen cycle that will help explain what is going on as your tank cycles.
Ammonia (from fish waste and your tapwater) is in two forms, Ionized and UnIonized. Unionized ammonia is toxic (poisonous) to fish. You can keep most of your ammonia in the Ionized state by keeping the tank temperature and pH low, within limits. [1] In a biofilter, Ammonia is eaten by Nitrosomonas bacteria. They turn the ammonia into Nitrite. Nitrite is bad for fish because it makes it harder for the fish's gills to take up oxygen from the water. Adding a little bit of aquarium salt helps with this. [2] Back in the biofilter, a second bacteria called Nitrobacter eat the Nitrite and convert it into Nitrate. Nitrate is generally safe for fish within reasonable limits. Nitrate can be removed by specialized filters, chemically, with plants, or by water changes.
The process of establishing these bacteria in your tank and biofilter is called Cycling. At a pH of 7.5 it takes 10-12 days for Nitrosomonas to fully colonize a biofilter. AFTER the tank has begun producing Nitrite, around day 8 or so, the Nitrobacter start to grow. They can take ~20-30 days or so to fully populate the biofilter. [3]
Does this mean you have to leave your tank empty (or with feeder guppies) for a month? No. You just have to use Chemical filtration (activated carbon or nitro-zorb) while waiting for the bacteria to establish. A temporary Activated carbon or Nitro zorb filter will keep your fish alive while the tank cycles.
In your original post, you mentioned that you were treating for Ich. Depending on what medication you used, you may have killed off the bacteria in the biofilter too. If that is the case then the cycling process starts over.
Good luck,
Ellie
[1]
Ammonia Toxicity in freshwater fish.
[2] For Tilapia in Aquaponics, you can go as high as 3ppt. Tilapia are pretty much indestructable, so do some research first.
[3]
Recirculating Tank Aquaculture Production Systems PDF the chart showing Ammonia/nitrite/nitrate levels over time is on page 5.