Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Given that it has currently Goldfish and turtles I would avoid this material from the point of view of possible pathogens.
If you dug up some garden soil that would be a better choice (if your garden soil would work in an aquarium) because it will not be exposed to water based organisms. If it comes from an active growing area dig deep enough that it does not have too much organic matter in it, or sift it so the larger particles are removed. Soil from under a lawn is suspect because of all the chemicals that may have been used on the lawn. If you have been in charge of it and you know for sure it is clean, then OK. A better choice might be some back corner of the garden under some bushes where nobody ever sprays anything, and any fertilizer has long been used by the plants.
Soil from a bag is usually good, if you get the right one.