The best way to store extra substrate is to get a large, glass container, preferrably (at least for me) in a rectangular shape. However, don't exceed 3". Store with liquid, but fill the glass container with water up to the top. If the container wasn't large enough to fit all of the excess substrate without exceeding the 3" limit, get another smaller glass container to hold the rest and again, store with water to the top. However, to keep the substrate in really good condition, it's best to have a light over the container(s), which stays on for at least 8 or 9 hours a day. And to keep the supposed bacteria alive in the Eco-Complete, make sure you keep some critters in the glass containers storing the substrate. Oh, and I also heard that including a bunch of plants with the substrate, water and fish is advisable. This is the *best* way I can think of to store extra substrate, but my husband doesn't seem to think so.
-Naomi