If you're doing water changes properly, you absolutely will not lose fish as a result. I can pretty much guarantee you that was not the cause of death..again if done properly.
Dont add any wierd additives that are supposed to absorb nitrates. Those are gimmicks. As a general rule, avoid any and all advice from the shops. The vast majority of them literally have no clue what they are talking about.
Vacuum the substrate thoroughly, doing a large water change. Temp match the new water and dose with a high quality dechlorinator like Seachum Prime. Use enough Prime for the entire tank volume and not just the water you are adding. This will neutralize the the ammonia and nitrites that are left behind for 24 hours, so they won't poison your fish.
By the test results you posted, which do not include ammonia, one of the most crucial to test for, I wonder, are you using test strips? Those are seriously inacurate. If they say your nitrite is at 3ppm, it's scary to think how high it could actually be, since 3ppm is already very severe. You need to be testing with a liquid test kit. Ammonia, nitrite, and nitrate are the most important tests to be running. Nitrate ideally should not exceed 20ppm, and nitrite and ammonia should remain at 0ppm. You need to continue doing daily water changes until they are down to these levels.
