90% of aquarium store workers that test your water for you have no clue what they are doing. Get your own test kits. You need at least Ammonia, Nitrite, and Nitrate test kits. After you get the kits, post your results.
Don't buy any new fish until we diagnose if there is anything obviously wrong with the water. Additional fish will make any nitrogen cycle issues worse.
How long has this tank been set up?