Mold is just a part of the biological process and won't hurt your frogs. Grape wood isn't at its best in humid tanks; it will deteriorate rather rapidly, but this doesn't make it harmful to your frogs at all. Cypress knee, cork bark and hard woods last longer, but they too, may grow some molds on occasion. Just wipe off the glass when any mold gets on it, and learn to enjoy the other molds that come and go. Eventually, you will have fewer of them, or they will at least be less conspicuous, as various algae and your plants take over.
If you learn to think of them as a normal biological cycling process, even necessary in the wild or in a relatively closed system as our dart tanks tend to be, they may not tend to drive you as crazy. Some of them can be rather pretty.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
6 D. galactonotus pumpkin orange splash back
7 D. imitator
6 D. leucomelas
5 D. pumilio Bastimentos
4 D. fantasticus
6 P. terribilis mint and organe
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus
2 P. lugubris