Testing your water regularly is the reason for an aquarist's success.
The main test that you should consider doing is NO3 (Nitrate). This is because Nitrates is a slow killier to fish. It affects the fish's gills and it slowly kills them.
Nitrites (NO2) and Ammonia (NH3/NH4) are fast killers to fish. However, you will visibly see when there are an abundance of Ammonia or Nitrites as your tank will appear cloudy.
You should also test your pH regularly to not get any surprises from shock.
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