Here's my cycling explanation.
Cycling is the process of developing nitrifying bacteria in the filter and on every surface of the tank. This bacteria is what is responsible for the biofiltration of a tank. Once established, this bacteria will convert ammonia to nitrite. Another bacteria will convert nitrite to nitrate. Ammonia is highly toxic to fish. Nitrite is less toxic than ammonia while nitrate is the least toxic of the three compounds. Normally, with a regular maintenance schedule, nitrates can be kept around 10ppm without much difficulty.
Here is what you need to know about cycling:
The cycle is started when fish are added to a tank. Fish give off ammonia from their gills through respiration. Fish also pass a dilute ammonia in their urine. Leftover food, decaying plant matter and other waste products will also give off ammonia. All ammonia is toxic at .25ppm or above. The amount of ammonia that it takes before a fish succumbs depends on the species. In a cycled tank there will be 0 ammonia.
It can take up to two weeks before enought nitrifying bacteria is established to bring the ammonia down to 0. Once this biofilter is established, it will remain in the tank and grow and decrease as you add and lose fish.
The bacteria responsible for the biofiltration is aerobic. This means that it loves and needs oxygen to survive. The majority of biofiltration in a tank takes place in the filter. This is why it is important to have a good quality filter with lots of room for sponges or filter floss. This will give the bacteria a larger surface area to colonize. The bacteria will also live on every srface inside the tank as well. They can't be uprooted by gravel vacuuming. The following is what can kill the bacteria:
1.Chlorine- never rinse your filter media in tap water. This will kill off the bacteria colonies. Rinse the filter cartridge, sponge, floss or whatever you are using in a bucket of used tank water and reuse it.
2. pH swings
3. poor filtration
4. temperature changes
5. daylight/light- this why filters are dark
6. medications such as antibiotics (regardless of what the packaging says)
After the initial two week period you will see a climb in nitrite. While nitrite isn't as toxic as ammonia it is still toxic-usually at 2ppm or above. To counter nitrite toxicity, add 1 teaspoon of aquarium salt per 20 gallons. Table salt is fine to use but it must be non-iodized. Water changes and gravel vacs must also be done to remove high nitrite levels.
Nitrites will remain very high for about six weeks. The bacteria responsible for converting nitrite is a painfully slow growing one. This bacteria are also sensitive to the things listed above.
One day you will find accumulating nitrate with falling nitrite. When the ammonia and nitrites both have a reading of 0 and you have a noticeable buildup of nitrate the cycle is complete.
Once complete, a regular maintenance schedule of weekly 20-30% water changes should be done along with thorough gravel vacs to remove fish poo, leftover food and other waste. This should be enough to keep your nitrates between 10-15ppm. If your nitrates are higher then you need to cut back on the amount of food that ois fed daily or increase the frequency of water changes. The lower the nitrates the more your fish will thrive.
One last note on nitrate. Keep your nitrate levels well below 20ppm, having nitrates above this can open the door to outbreaks of flexibacter and other nasty pathogens.