If you are sure its the tank then it has to be build up from the water. You might try flushing it first before replacing it.
or
Sanitizing instructions:
It may be necessary to sanitize ROPRO tanks from time to time.
A 5% Clorox®bleach is an acceptable sanitizing solution, along with similar brands. To sanitize tank, flush system with chlorine, without any filters, and allow the water and chlorine solution to flow into tank. Discard the first two tanks of water produced as they contain chlorine. Do not use this water. Do not directly expose tank to high concentrations of sanitizing chemicals. Properly dilute bleach with water to prevent damage to tank and diaphragm.
You might run some declor thru it after the sanitizing.
I would just replace it