I do believe that you may have a problem with ammonia in your tank. Ever heard of cycling?
See, it's crucial that your tank cycles in peace (better without fish) so that the beneficial bacteria can settle to deal with the waste your fish produce
I'm talking about the nitrogen cycle, and this is where ammonia is turned into nitrite and nitrite is turned into nitrate.
If you want to save your fish, you need to buy yourself a test kit for these parameters asap! They're usually quite affordable since they last for a long time! Buy liquid tests, as test strips usually aren't accurate. So, buy yourself a good test kit with tests for pH, ammonia, nitrite and nitrate.
Now, when you've got the test you need to test for ammonia (and the rest). If this is high you'll have to do water changes everytime you notice it going up! Test every day, and change about 20% every time it reaches dangerous levels.
After a week or two you should notice nitrite showing on your scale, this is when the first bacteria have established! It also means that it's safer for your fish
But you should still test every day and change 20% as soon as the levels are too high.
When the nitrite is going away you should notice nitrates, and from now on, as long as your tank is properly stocked, you can change water weekly
Test for nitrate and pH once a week and change water accordingly. The higher nitrate, the bigger water change! Change at least half of the water at a time or more.
Finally, I would like you to answer a few questions.
- How big is your tank?
- For how long have you had it?
- How many fish?
- What kinds of fish?