Do I need to adjust my Specific Gravity?

[jassz]

Hi all, newbie here.

I just bought/moved an existing FOWLR tank last weekend. The previous owner apparently kept the SG at 1.024. We moved 100+ gallons of exisiting water (it's 135g with sump), and the new water was also tested at the time we added it (I'm not exactly sure what it was, the prev. owner checked it). So I'm quite baffled as to why now, not even a week later, the SG could drop to 1.019. I did change out about 15 gallons a couple of days ago (the nirtrate was climbing), and the SG of that change water was about 1.019, but I figured that 15 gallon wouldn't drop the overall SG. It doesn't make sense it could have dropped, so I'm a little suspicious of the hydrometer.

But if my SG really IS 1.019 (the readings are consistant), is it harmful to the fish or rocks to be at this level? Most fish seem fine, a couple seem lethargic but I don't know what they are normally like. What advantage would there be in bringing it up? How do I bring it up? I mean, I know you add salt, but how on earth do you gauge how much? From what I've read bringing it up too fast could be problematic.

Thanks for your help!