You can’t just put fish in your aquarium right after you fill the tank up with water. You have to cycle your tank. Cycling is a process that can take anywhere from two weeks to two months. You do this to grow something called beneficial bacteria which can grow in your substrate/gravel. You need to do this if you want an established aquarium. Something helpful I recommend is API freshwater master test kit which tests the amount of ammonia, nitrate, nitrite, ph, kh, gh, and phosphate. Ammonia is like poison in the aquarium. This is the first step of cycling. If you test your water, you’ll notice some ammonia in the tank. This isn’t bad, trust me. Just don’t put any fish in the aquarium. A little liquid ammonia can help cycle your tank too. You should also put filter media in the tank too because beneficial bacteria will grow on it too. You should be testing your water for ammonia every other day. Soon, you’ll notice as your testing the ammonia will be decreasing. This means the nitrIte levels, NOT nitrAte levels, will be increasing. You should start testing your nitrite now. Nitrite is like poison. Nitrite levels will become higher and higher. Then, you will notice it decreasing a little. This means nitrAte levels will be increasing. Test your nitrate now. Nitrate isn’t as dangerous as nitrite and ammonia. You should have under 40 ppm of it. This means beneficial bacteria has grown. Beneficial bacteria converts dangerous nitrite into nitrate. This is the purpose of it. Now, you can put your filter media back into your filter and gravel will have beneficial bacteria in it too. The cycling process has finished.