Hi and welcome to the forum
How long has the tank been set up for?
Is there a filter on the tank?
How often do you do water changes and how much water do you change?
How often are you feeding the fish?
What do the fish look like when they die?
Are there any funny colours, patches, spots or markings on them?
What fish do you have in the tank, including the ones that died?
Can you post a picture of the remaining fish or any fish that look sick?
When you get water tested at a pet shop and they say "it is fine" ask them what the results are in numbers and then write the results down.
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Right now you should stop feeding the fish and do a 75% water change and gravel clean the substrate each day until this is resolved.
If the tank has been running for more than 6 weeks you can clean the filter if it hasn't been done in the last 2 weeks. If you have a power filter, wash the filter materials in a bucket of tank water.
Take pictures of the sick fish and post some on here. Make sure the pictures clearly show any marks or issues.
If you can get the water tested again for Ammonia, Nitrite, Nitrate & pH, and post the results here, it would help.
Do Not add any new fish or plants at this stage. Just do daily water changes and get some pictures on here asap.