Give us more info on your tank and set up, stocking, etc. Some fish like alkaline water.
Did you do a PWC or large water change recently?
What is your tap/source water baseline? To establish this, you need to fill a bucket or jug with a gallon of cold water (run the water for a minute first to get stagnant water out of your home pipes). Test the tap/source water for all tests that you have.. preferably ammonia, nitrite, nitrate, pH, GH and KH. POst your results.
Add your dechlor product. Wait 48 hours and test it again. Post your results.
This will give you and us an idea of what is happening to your tap/source water. Then we can try to figure out if it's something in your tank that might be leeching into your water column.
What kind of substrate do you use? Any real rocks as decorations?