8 to 9 I think on average is best. If you start going over 10 you'll get alk burns on some of the sps corals. I have read however that some salts have higher levels of metals such as boron which actually gives a false Alk reading on test kits and if you're using such salt you need to maintain higher Alk levels around 11-12 to achieve actual alk levels of 8-9. Not sure on all salts but I know Ocean Pure is one of them. I believe the "best" alk level will depend on the salt you use and the types of equipment you employ, a little experimenting is necessary to determine what is best for your system.
I also seem to recall that natural seawater usually doesn't exceed 8 dkh.
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