Concerning the garlic, check out a recent thread I posted about garlic, it references and gives a link to an article about it. As far as your specific gravity, when I was keeping fish only tanks, I kept my SG at around 1.017. This lower salinity allows a higher oxygen content, and also is less desirable to parasites like ich, as they don't survive well in lower salinities. I kept mine there permanently without ever experiencing any problems. Don't do this with inverts, though, you should keep their SG at 1.0235 like sea water.