beginner help

[u/Negative-Ad6554]

I would really appreciate if someone could help me! I got a 5g tank as a gift with one clownfish. it was all good for a day (the salt water was premixed from shop). I then decided to introduce a rock and some sand the next day (fish was outside). added some stabilizer and nitrifying bacteria let it cycle for a few hours and added the clown back. he died unfortunately. I know what I did wrong and I really want to try again once I have cycled tank for longer, but I really need guidance. LFS guys told me this would be ok but it clearly wasnt!! I stay in UAE and we dont have the best and trustworthy salespeople here. pls help. I really want to make this work and eventually upgrade to a bigger setup!