You need to be able to test for ammonia and nitrite. Nitrate is less important at the beginning, but useful. Many of us use API 'master' test kits which contains testers for ammonia, nitrite, nitrate and pH. There are also 5 in 1 or 6 in 1 strip testers but these don't include ammonia so testers for that have to be bought separately.
In a tank that was not fully cycled before fish go into the tank, ammonia will show up almost immediately. This burns the fish's skin and gills, and burns to gills make it harder for them to get oxygen from the water. After a few weeks, enough micro-organisms grow to 'eat' this ammonia and they 'poop' nitrite. This also harms the fish as it binds to their blood stopping the blood taking up oxygen - basically it does what carbon monoxide does to us. After a few more weeks, more micro-organisms grow which 'eat' nitrite and 'poop' nitrate, which is less toxic than the other two but has long term effects if it's too high. We should aim to keep nitrate below 20 ppm, and the lower the better.
In an uncycled tank, testing every day for ammonia and nitrite is important so we can do a water change when either read above zero. At first only ammonia will read above zero and it could take a few weeks before nitrite shows up.
In a tank that was not fully cycled before fish go into the tank, ammonia will show up almost immediately. This burns the fish's skin and gills, and burns to gills make it harder for them to get oxygen from the water. After a few weeks, enough micro-organisms grow to 'eat' this ammonia and they 'poop' nitrite. This also harms the fish as it binds to their blood stopping the blood taking up oxygen - basically it does what carbon monoxide does to us. After a few more weeks, more micro-organisms grow which 'eat' nitrite and 'poop' nitrate, which is less toxic than the other two but has long term effects if it's too high. We should aim to keep nitrate below 20 ppm, and the lower the better.
In an uncycled tank, testing every day for ammonia and nitrite is important so we can do a water change when either read above zero. At first only ammonia will read above zero and it could take a few weeks before nitrite shows up.