There are two answers to that question, the first covers what you need when you first set up an aquarium and the second covers what to so once the tank is set up.
Whether it is a new or established tank what you are trying to do is maintain the water quality at a level which is best for your fish so the first thing to do is understand the sort of conditions that your chosen fish prefer.
All fish will live happiest between certain water parameters and this varies with each type of fish.
In a new tank you need to be able to identify when the nitrogen cycle has taken effect, in other words when your tank has cycled and it is safe to start adding fish..
The biological filter is not yet established so any fish waste or undigested food would simply lead to an Ammonia Spike.
This is the first stage of the Nitrogen Cycle and can be easily detected with an Ammonia Test.
As bacteria start to breed you will see the Ammonia Spike replaced by a nitrite spike as the Ammonia is converted to nitrite by the first set of friendly bacteria, Nitrosomonas.
This is stage two of the Nitrogen Cycle and can be tested for with a nitrite test As the tank finishes its cycle you will see the Nitrite spike replaced by a Nitrate spike as the Nitrospira bacteria start to breed and feed on the nitrites converting them to less harmful Nitrates.
This is the third and final stage of the Nitrogen Cycle which will show up through a Nitrate Test.
Both Ammonia and Nitrite are harmful to fish so it is only when nitrite levels have fallen back to their original levels that it is safe to start adding fish Make sure that you add the fish slowly, a few at a time, allowing the nitrogen cycle to catch up each time.
Test the water every couple of days while you are adding fish and carry out a partial water change (up to 30%) if there is an Ammonia or Nitrite spike.
This cycling process can take several weeks and you cannot tell which stage you are at without testing the water.
It is essential to test the water quality of your freshwater fish tank throughout the nitrogen cycle.
So the three test kits that you definitely need simply to monitor the nitrogen cycle are Ammonia, Nitrite and Nitrate.
The fourth that you need is pH to test the acidity of the tank.
Whenever you are introducing fish you need to make sure that there isn't too big a difference between the pH level in your tank and the pH level of the tank that they came from.
If there is, you need to acclimatise the fish carefully to avoid shock and stress.
Once your tank has matured, the testing slows down.
You still need to test your water regularly and make a note of the results so that you are aware of any changes before they have the chance to turn into problems.
Make it part of your routine to test the water before you carry out your regular water changes, and definitely if your fish are acting differently.
One of the biggest problems in a mature tank is nitrate.
Yes, it's far less dangerous than ammonia and nitrite, but the trouble with nitrate is that it often begins to rise over time.
Your planting and regular water changes will go a long way towards dealing with nitrate but it is still worth monitoring on a monthly basis.