How to clean out fresh water tank for salt water

DON'T use water!

OK, you can use water...

Seriously though, a new/ rinsed scotch brite pad and some vinegar will do the trick... I have vinegar in a spray bottle for this purpose... as far as cost it's like a dollar more than bleach.... I use it to clean the outside of my tank all the time.

Using lots of bleach can be dangerous to your health/ clothes/ house/ etc. I prefer to only use bleach when I have to and use vinegar for everything else.

I also like vinegar also because you can add baking soda to your rinse to neutralize it.

you can use either as long as the tank is rinsed clean and there is no residue of bleach or vinegar, though that's probably a given.