New 55g, RO/DI water questions

I am setting up a 55g tank in within the next week and I had some questions about using RO/DI water. Until now my fish have been in a 20g and I have been using tap water for everything since it already has a ph of 7.9. I plan to use only RO/DI water in the 55g and would like to know what I need to add back into the water to replace what was removed by the RO/DI unit? The tank will be a FOWLR and have various inverts.

Also, I have read a lot about how inaccurate hydrometers are and was wondering if I need to get a refractometer. As mentioned above there won't be anything but fish and inverts unless they come in on the LR, so I'm not sure how precise I need to be.

Thanks for the quick replies. I have read various posts that mention adding calcium, iodine, or various minerals that are in tap water, so I wasn't sure if that was necessary, especially when replacing evaporated water between changes.

Pallen81 said:

if you don't use Reef Crystals, make sure to buffer the RO water with some marine pH buffer.

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I have about 100 gallons worth of Instant Ocean sea salt, so is that considered good? If it is good and I use it will I need to buffer the water first? I did a test a week ago on some RO water that I get from WalMart for the freshwater tank and adding salt makes the ph go from low 6's to low 8's.