Oceanic salt for fish only setup ?

Ermm... I wouldn't say there is much difference from what I've seen. I wait several days before adding water, but not because of salt. The longer you let water sit after dechlorinating and mixing, the more undesireable trace elements in the water get neutralized. If you're using RO water, I don't think you need to wait a full day with a normal salt mix unless you are doing a large water change.

Maybe I'm misinformed on the RO process, though.
 
I hear really good things about Oceanic salt. One of the reefers here says that his SPS slime a lot less after water changes since converting from Instant Ocean. I agree with Ben that dissolving isn't the only issue. Remember that the buffering in seawater is based mostly on carbonate, so the water needs to be in equilibirum with atmospheric CO2 before the it will be correct.
 
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