, I explain what the Nitrogen Cycle is.
Fish eat and they produce waste ( poo, etc ) plants have leaves which fall off and decompose.
Fish waste and decomposed plant matter causes Ammonia to show up. This is very toxic to fish. It is as we would swim in our own sewerage.
To get Ammonia down we need bacteria which brake it down into Nitrite. This is still toxic to our fish. So we need bacteria to brake this one down into Nitrate. Nitrate is good for the plants to grow and in lower concentration it is harmless to fish.
To know how much Ammonia, Nitrite and Nitrate you have in your water you need your test kit. When the readings for Ammonia and Nitrite are 0 and Nitrate shows your tank is cycled. Nitrate shoudn't be above the 35-40 mark. If it is a 50% waterchange is necessary.
Waterchanges are also necessary as long as Ammonia and Nitrites are showing and in this case daily.
As you surely noticed, it is a good thing to have live plants in your tank. They use those Nitrates, also they need Carbondioxide the fishes produce for their photosynthesis and release oxygen back into the water for our fishies to breathe.
Hope this helps
Please keep asking if there is sth you don't know, we are glad to help