I agree test for everything. How old are the filters/membranes in your RO
unit? If it gets worse after every water change despite tests and TDS
showing up good something has to be getting by? Did something get into your water storage? I use a trash can for mine and people constantly throw trash in it even though it's full of water...
I think autumnsky has a really good idea. If you can change salt or even just get new of the same that you like? I'd even say buy some water from another source if you can and do water changes with that.
I'm sorry this is happening. I hope we can help you find a solution
Sent from my iPhone using Aquarium Advice