quarantine isolation

I keep losing my fish in qt. Treatment is Cupramine and sometimes PraziPro. I don't use Prazi but once a week if I use it. My fish will be fine for a week and then they just crash. Ammonia not 0 but .05-.25. Ph is 8.2. Salt 1.026

I use my Hanna Salinity Tester routinely, but would like to check my quarantine tank salinity. I've asked Hanna, but thought someone here may have information on safe methods to disinfect without harming the carbon electrodes. Thanks in advance.