Once here you should tell the people working in the shop that you’ve got a new fish tank and want to know if the water is ready for fish to be introduced. This shouldn’t cost you anything. What they’re testing for is traces of ammonia nitrate, and they’ll be able to tell you when your water is ready to receive the fish. If your water passes the test and the store tell you it’s ready for the fish then start with no more than five fairly hardy fish (guppies or platies etc). These should now be able to stand the water in the new tank though it won’t yet be 100% how it should be. Don’t drop them straight into the tank, but place them in in a bag of the water they came in so that they can get used to the temperature. After ten minutes allow them to gently get used to the different water and let them loose in it. Do not add any more fish for an additional 6 weeks as the water won’t be able to stay in balance. When you do, make sure not to add more than 6 fish in at a time.