Just to add to what vince has said I also raise my springs on coir/orchid bark. Lots of people do raise them just on pieces of lumpwood charcoal. To get them off the charcoal pieces just wash them off the charcoal with spring/dechlorinated water and pour the water in your viv. I personally prefer the coir/orchid bark - kept in 2L ice cream tubs and fed veg peelings/yeast/tropical fish flake/moistened instant oats (when feeding yeast make sure you open the cultures daily to stop CO2 buildup).
There are lots of other livefoods that you can use as a standby in case of fruit fly cultures crashing - I have done an article about it thats in the articles section:

Frog Forum - Culturing Livefood for your Dartfrogs

You'll always get some fly culture crashes so I have found its good to get in the habit of doing new fly cultures every week. Keep them on either mite paper or shallow trays of water to help keep mites down.