i wouldnt ever suggest feeding an outside bug to a captive animal' fertilizer free or not. They carry bacteria, microbes, and parasites that are foreign to a captive bred animal. when looking for worms at shops make sure they are not scented. I've been trying to find nutritional benefits between using european night crawlers and red wigglers but have not been able to find any info. I would only assume that night crawlers are better because they are fat, but the down side is that they compost and reproduce slower than red wigglers. it really depends on the size of what you are feeding. i think my horticulture professor mentioned that a cup of red wigglers can decompose a pound of kitchen scraps per week. pretty cool stuff. I assume that gut loading them with potatoes and carrots would be most nutritional for your froggy friend