Planted Tank Guru
Join Date: Sep 2006
Location: Northern California
If I were you, I'd start to cycle the tank now, and then when the Eco-Complete comes in, drain the water into a bucket. Add the Eco-Complete, add enough water to get to ~5 gallons, do a small WC to get rid of debris, then re-add the old tank water.
Run your filter with a bag of carbon just to clear up the water some. It'll last 2-3 weeks. After that, don't use any more carbon.