Just add it-- no issues for your tank. It's actually designed to be at least partially replaced every couple of years. I started my tank with it, and recently replaced the entire amount after the ice storm (what I'd had sat stagnant in sump for the 9 days, and I didn't trust it). No problems.
I started my current tank in September/October of '05 with the ecosystem/miracle mud sump and have NEVER had any significant algae issues (actually, the ONLY algae issue I've ever had in it is some on the back wall, where the flow is low)... and that's counting the most recent power outage (9 days), the microburst power outage (4 days), and a few other shorter outages. I definitely think the Miracle Mud is a great choice... add a good skimmer, some macro, and enjoy.