I've never used eco complete but I know the bag says it contains live bacteria. Doing water changes will not remove enough bacteria to cause a problem in your tank. Water changes at a later point are essential to keeping a "balance" in the aquarium. The majority of the bacteria is attached to the substrate itself and any highly oxygenated areas in the tank.
Basically the majority of the bacteria will live in the filter and substrate/glass.
If you want to clear up the water a bit feel free to do water changes, it will also help remove excess nutrients in the water column. 2 or 3 50% water changes will most likely clear everything up(just be cautious to add the water slowly into the tank so you don't stir everything up again.. you can use a plate to pour the water on or a Tupperware or something). Make sure you are dechlorinating the water with dechlorinator(prime seems to be the dechlorinator of choice)
Once you have your water all cleared up you will need to let the tank cycle. There are many methods for cycling a tank but I think the fish less method is the most humane and will cycle the tank just as fast as using a fish without causing damage to the fishes gills or killing them.
You can read up on it here: http://www.tropical-fish-success.com...k-cycling.html
Basically you will need to add a source of ammonia, sometimes ammonia appears almost out of nowhere(you will probably get snails on your plants you buy and they will start breaking down plants which are decaying and producing waste..very small amounts of ammonia). Or you can use janitorial grade ammonia which is just water and ammonia, no additives. If you shake it and there are bubbles you cannot use it. You will need a test kit to figure out how much ammonia you are adding to the tank.
During the cycling process I tend to just do top offs(fill up the water that evaporates from the tank) but I do not do water changes because it can remove the ammonia/nitrite the bacteria are using to grow.
Once you have ammonia, then it will be broken down to nitrite, and after that to nitrate by different colonies of bacteria. The process takes around 16-30 days depending on how you are cycling the tank. It will be shorter if you add filter media from an established tank into your filter...etc(read the link :P)