Different fly places will add different stuff to the media to try and place theirs above the rest(color enhancers for the frogs, vitamins, algae, etc.). The important thing is to find a media that has a mold inhibitor. The majority of media sold online will have this in it (the stuff I sell does, saurian's does, Ed's does, etc.). Keep in mind that how you make the media will be different depending on your climate (here in michigan we have very humid summers so i use less water, whereas if you were in arizona where they have almost no humidity you would need to add more water. I have found that when cultures are dry they are more suseptible to mold).
To cut down on mold use vinegar (I think it makes the culture smell unbearable) or use a media with a mold inhibitor in it. Also keep them away from your AC unit as they are notorious for spreading mold spores.
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Josh Willard
www.joshsfrogs.com