The first thing to take into consideratiom is tank size. Always buy the biggest tank you can afford and is practical for your space.
Most pet stores will sell you any fish you like with little to no advice pertaining to the fish's needs. This article will help you make the right descisions when choosing fish for your aquarium.
There are many fish normally found in pet stores as juveniles, cute and small. But what you probably don't know and neither do they is that quite a few of them get very large. I know its hard not to be impulsive when at a pet store but the right thing to do is pick a number of fish you like and go home and look them up to see how big they get and what kind of personality the fish generally have.
There are a lot of fish to choose from and quite a few of them don't go well together.
In freshwater aquariums it is normal to hear things about having the ideal water parameters for each fish, i.e. Ph, KH, GH and Tepm.
Truly most fish in the aquarium hobby are tank bred so only one of those parameters is really necessary to keep in mind when choosing fish to keep together. Temperature.
It is always much easier for you and the fish to keep them in water the way it comes out of your tap then to try to adjust the PH, KH or GH.But a fish that lives and thrives in cold water will most likely die and/or get sick in warmer water. The same goes for warm water fish.
Fish Size/Tank Size
It is just guidline to only have about 1 gallon to every inch of fish. But the larger the tank the looser the rule applies. 10 fish 1 inch in size are fine in a ten gallon tank but in a 100 gallon tank you can easily keep 200-300 one inch fish. It is up to the aquariest to use his/her discretion when stocking a tank.
Keep the water clean with a power filter, do 25% to 50% water changes atleast once a week, Always add water dechlorinator to the buckets of water before adding it to the aquarium, Use a heater to keep the water temperature stable, Clean the media in the filter once a month, Always use tank water to rinse the filter media(never tap water), feed your fish 2-3 times a day until full grown then once a day is fine and know weather your fish are herbivores, carnivores or omnivores and feed them accordingly.