as Byron suggested, check the tap water for ammonia, nitrate and pH.
not sure about your water company saying fluoride is a naturally occurring chemical in the water. cough cough bullsit.
If your tap water has no ammonia or nitrate then simply do bigger water changes and gravel clean to dilute the stuff in the tank. You can do a 75% water change every day without any issues as long as the new water has no chlorine/ chloramine in. And if your tap water is clean (has no ammonia or nitrate) then I would do a 75% water change each day for a week simply to dilute everything in the tank.
The same thing if you ever lose a fish, do a 75% water change and gravel clean to dilute anything in the water.
Just make sure any new water going into the tank is free of chlorine/ chloramine before it is added to the tank.
If possible use a clean fish only bucket that does not get used for anything else. Fill the bucket with tap water, add the required amount of dechlorinating agent, you can add a bit more than stated without any issues. Then aerate the bucket of water for 30 minutes or longer before adding to the tank.
If you are having problems with the tank, then make up a new bucket of water after each water change, add dechlorinator and aerate until it gets used the following day.
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If the aquarium is kept in your bedroom, make sure you don't use any hair spray, perfume, deodorant, or anything else that creates fumes. These can all get into the tank and stress or kill fish.
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if you get any more sick or dead fish, get a photograph to post on here, do water test and post results on here with the picture of the fish, and do a big water change and gravel clean