The first few times with a new salt mix I test for pH, alkalinity, nitrate, calcium and phosphate just to be sure those levels are within acceptable ranges but once I've used the salt for awhile I only test specific gravity.
Glad this topic came up actually, I recently experienced the downside of this. I always test the first few batches of new salt for quality and then assume things will be normally as well. This proved fatal for my 55g tank in the last few days. Something, as of yet undetermined, happened to my salt in the past few weeks and when I did a large water change and went to bed, but by morning something was very wrong. I immediately thought some form of contamination and used a new bucket and hose doing another immediate water change. Little did I know I made things worse.
Soon after by recommendation I tested for alk
and learned that the salt which was producing and alk
of around 10dkh was now not even registering on my test kit. The tank went through a massive alk
spike and the resulting PH
crash and calcium precipitation. I lost every single coral in my tank, luckily my fish were stressed but survived.
I'm still working on finding the cause of the problem with the salt, but it is safe to say that anyone using buckets of salt should consider testing every few water changes to ensure the proper parameters. My bucket was stored as I have been for years, tightly sealed and in a small coat closet away from chemicals, etc.