1st Tank Fill

[FishyFishy!]

I generally use RO/DI. I have rarely had to drain it because of a leak. Usually (if your plumbing is done right) you can remove that section of plumbing using unions and valves without having to drain the entire tank. This gives you the option to fix the leak, re-test, and then add salt without wasting water.

If you are just testing the tank seams for leaks (if you don't run an overflow system) I would still use RO/DI and risk the having to drain it factor.

However if you are not planning on setting the tank up for a while after your leak test.... I wouln't bother wasting the RO/DI water. Just use tapwater if you plan on draining it anyways.