is specific gravity (that is measured, albeit somewhat inacurately, by your hydrometer). 1.021 is on the low side and most keep their SG
closer to 1.023-1.025. As long as you're watching your SG
in your main you should be ok. SW
is an acronym for Salt Water (if you hover over the red links within a post you'll see the hint text for the acronym).
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