What am i doing wrong?
> fish tank 20 gallon
> water filter (2) / (1) 10 gallon & (1) 20 gallon
> tetra whisper air pump for a 40 gallon tank
> tank has lighting

why do my fish keep dying? i did everything from changing all the water to changing only 1/3. i've notice someone mention something about cycling?

When you are starting a new tank you must allow the tank to cycle before adding to many fish. You should add no more then 2-3 small fish so they can lay waste inside the tank to produce bacteria. This bacteria allows your tank to cycle and stabilize your biological filter. I would wait approximately 1 month before starting to add more fish. Then after that time I would add a few fish every week. If you add to many fish early without the tank being properly cycled the nitrate levels inside the tank will be so high from the waste of the fish that it will cause the fish to die. I hope this helps and good luck with your fish tank. If you have any other questions just send me a message.

Ditto on the previous post.

Just want to mention one other thing. While your tank is cycling, the only way to remove ammonia and nitrites is to do frequent water changes. Buy yourself a kit that tests for these chemicals and change as much water as necessary to keep levels low. Remember, ANY amount of ammonia in your tank is harmful to fish. It won't be possible to keep ammonia at zero until your tank is cycled, but meanwhile, aim for .25.