What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0. Mollies need a GH above 250ppm the other common livebearers are fine with a GH around 200ppm.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
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How often do you feed the fish?
How often do you do water changes and how much do you change?
If you put a lot of food into the tank or don't do many water changes, you reduce the number of fish that can live in the aquarium.
Common livebearers (guppies, swordtails, platies and mollies) breed readily in captivity and a female can produce 50-100+ young per batch, which might happen every 4-6 weeks. If you plan on keeping female livebearers, you need to make sure there is space for the babies.
Do not keep swordtails and platies together because they hybridise. Choose one or the other but don't have both in the same tank.