The second thing you should do is get a good water testing kit and learn to use it. I use the API Freshwater Master Test Kit. It tests for pH, Ammonia, Nitrate, and Nitrites, and one kit lasts a long time. There are also test strips you can use, but the kit that comes with the little test tubes seems more reliable to me.
You need to monitor the chemical makeup of the water to be sure it is safe. The water testing kit tells you when your tank is cycled correctly, and then lets you monitor the conditions in the tank as you add more fish.
Adding fish impacts water conditions, and the more fish you have, the higher the demand on the healthy microorganisms (bacteria) to get the job done, breaking down their waste.
The living inhabitants of your tank are collectively referred to as the bioload. Everything that breathes, eats, and excretes in your tank contributes to the bioload, from fish to frogs to shrimp. Even those rotten little snails you can’t get rid of put strain on your tank’s ecosystem.
Once your tank is up and running for a while, the bacteria colonies in your tank will have matured to where you probably won’t need to test the water all the time, unless something strange happens like a sudden die-off of your stock. But in the beginning, you want to get a handle on your water conditions and make sure you aren’t overstocking
You need a kit that measures ammonia, nitrates, nitrates, and pH. Your ideal water parameters are: Ammonia=0ppm, Nitrites=0ppm, Nitrates<25ppm, pH=neutral, or close to it.