When you start a tank w/ everything clean and fresh water your tank has to go through a cycle to balance everything, ph, amonia, nitrates, etc. The cycle doesn't start until you add something living, fish. It takes a little over a month to complete the first cycle. Everything will spike up and down throughout the cycle. This is what makes it hard on the fish. The more fish you add the more dangerous it becomes. This is why so many people that set up new tanks have so much trouble. They want to fill it with fish all at once.
For new tanks: Set up everything fill and run for a few days to make sure there is no problem with the system. Purchase 1 fish and wait a month. Don't test water or do any water changes until month is up. Test water to see if initial cycle is complete. If it is you are safe to add the rest of the fish. If not wait another week and test again. Now that the cycle is complete and you have added the fish, replace no more than 1/3 water every couple weeks. Never empty tank all the way or you will be starting from scratch. Never use cleaner of any kind on any part of the tank or anything that goes in it.
Max size of fish for tank, pick w/ full grown size in mind. For tropicals 1 inch per gallon. One neon uses 1 1/2 gallons. Cichlids 1 inch per 2 gallons of water and goldfish at 1 inch per 5 gallons.
Make sure you have a good filtration system and heater if needed.
Betas are one of the only fish that can or should ever live in a bowl, because they can come up for air.