Tank is 37 gallons, I use the reccommended amount, so take how much I pulled out and divide by the reccommended amount and that’s how much I put in in mL, I let it sit for around 15-20 minutes.You need to look up your water parameters on your suppliers web site, there is something else going on here. Clean fresh water will not kill fish..
How big is the tank.
When you clean it tell me exactly what you do.
How much dechlorinator do you use
How long do you let the new water sit before adding it to the tank? Yes yes manufacturers of dechlorinators claim they instantly deal with this or that but I dont trust them.