My suggestion to avoid potting soils with an added fertilizer (especially for one's first foray into using soil underlayers) was based on the El Natural methodology - the ideal soil will have nutrients, but in moderation. Just as the addition of composted manure is also not advised, the additional fertilizer in supplemented potting soils is considered excessive for an El Natural setup. In this type of setup, fertilizers are not added straight to the water column, either - the addition of fishfood, and its degradation by bacteria provide the main source of macronutrients for the plants.
There were a few cases where folks used a fertilizer-enriched potting soil in an El Natural setup, and they had great difficulty with their tanks, many times greater than the "usual" problems encountered as an El Natural soil underlayer is "settling in".
If the parameters are changed, such as the addition of CO2 supplementation or water column fertilization is added, then Miracle Gro potting soil with fertilizers might be just the right thing. But that's a very different setup.
-Jane |