I just logged on to ask how often people completely tear down and redo their tanks for exactly this reason. I have a 55 gallon tank with miracle gro organic potting mix topped with eco-complete. It is just about a year old and moderately planted with not very many fish. I'm redoing mine this week for a few reasons:
1- I was worried about the organic matter in the substrate
2- I was worried the plants had used up all of the nutrients in the soil/eco-complete
3- I want to do a major rescaping of the plants
So based upon what I've read here, one year is not a long time with healthy plants and regular water changes?