it depends on the size of the tank, what you are keeping, and how much you are willing to spend.
I use EVC 2 part with a Aquamedic pump. I have it set to dose 4 times a day. A larger tank than mine (55), 2 part would get expensive, so either make your own, or use a reactor.
here's some links:
http://www.advancedaquarist.com/issues/feb2003/chem.htm">http://www.advancedaquarist.com/issues/feb2003/chem.htm</a>
[IMG]http://www.reefkeeping.com/issues/2004-05/rhf/index.php">http://www.reefkeeping.com/issues/2004-05/rhf/index.php</a>
[IMG]http://www.advancedaquarist.com/issues/nov2002/chem.htm">http://www.advancedaquarist.com/issues/nov2002/chem.htm</a>
[IMG]http://www.reefkeeping.com/issues/2004-09/rhf/index.php">http://www.reefkeeping.com/issues/2004-09/rhf/index.php</a>
Ralph