AMMONIA
Why test for ammonia?
Tropical fish continually release ammonia directly into the aquarium through their gills,urine,and solid waste.Uneaten food and other decaying organic matter also add ammonia to the water.A natural mechanism exists that controls ammonia in the aquarium-the biological filter.It is made up of nitrifying bacteria,which live in the gravel bed.However,as with any natural process,imbalances can occur.Testing for the presence of toxic ammonia is essential,so that once detected,steps can be taken to remove it.Otherwise,ammonia in the aquarium will damage gill membranes,and prevent fish from carrying on normal respiration.High levels of ammonia quickly lead to fish death.Even trace amounts stress fish,suppressing their immune system and thebery increasing the likelihood of disease outbreaks and subsequent fish loss.

PH effect
6.8ph----(0 ppm to 3ppm =safe)(6 ppm= stress)
7.2ph----(0 ppm to 1ppm =safe)(3 ppm to 6 ppm = stress)
7.6ph----(0 ppm to 0.5ppm=safe)(1 ppm to 3ppm =stress)(6 ppm = slow death)
8.0ph----(0 ppm to 0.25ppm =safe)(0.5 ppm to 1ppm =stress)(3 ppm =slow death)( 6ppm =rapid death)

water changes
Changing 25% of the aquarium water will reduce the ammonia level.In a emergency,a daily water change may be required over several days.

NITRITE
Why test for Nitrite?
Nitrite is produced in the aquarium by the biological filter.Benefical bacteria in the biological filter convert toxic ammonia into nitrite(also toxic).The biologial filter then convers nitrite into nitrate .Testing for the presence of toxic nitrite is essential,so that once detected,steps can be taken to remove it.Otherwise,nitrite in the aquarium will prevent fish from carrying on normal respiration.High levels of nitrite quickly lead to fish death.Even trace amounts of nitrite stress fish,suppressing their immune system.
Making partial water changes can also help reduce the nitrite level,specially if the initial nitrite levels is very high.Adding Salt will reduce nitrite toxicity to fish while the biological filter is removing the nitrite.

nitrite levels
0ppm to 0.5ppm =safe
1.0ppm to 3.0ppm=stress
5.0ppm to 10.0ppm= toxic

NITRATE
Why test for Nitrate?
Nitrate is produced in the aquarium by the biological filter.Benefical bacteria in the biological filter convert toxic ammonia and nitrite into nitrate.A high nitrate levels indicates a buildup of fish waste and organic compounds,resultingin poor water quality and contributing to the likelihood of fish disease.Maintaining a low nitrate level improves the health of fish and invertebrates.Excessive nitrate also provides a nitrogen source that can stimulate algal blooms.Aquarium water should be tested for nitate once a weekto make sure the nitrate does not reach an undesirable level.

nitrate levels
0 ppm to 40 ppm=safe
80 ppm to 200pmm=stress

Making partial water changes can also help reduce nitrate,especially if the levels is very high.Check your tap water nitrate will be recomended because many tap water supplies contain nitrate,it can be difficult to lower nitrate levels by this method.

I hope this information can help you guys.

Excellent writeup.