You might just need some magnesium. You can check into adding some dissolved plain, nothing added pharmacy Epsom Salts which is magnesium sulfate, and possibly some calcium powder.
If you can do a GH
test and see what you find mainly with the kH
it could be very useful.
What kind of dechlorinator are you using? Some types use different methods of binding the chlorine and if you already have low KH
and you use a type which uses calcium to bind with it, you can crash your tank. Personally have had that terrible experience. Others use O2
. And I am not a chemist so my knowledge is limited and the intricate details of all the ways those things work.
I use Prime and previously tried a different type dechlorinator, which worked great for people with harder water and killed fish when people had very low KH
This is a really thorough article re: water chemistry
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves