First, let me say that everyone raises dart frog tadpoles differently. That being said...

I think the mold is there because the tadpole tea stayed in contact with fresh air too long. I've raised dozens of D. Leucomelas and Tinctorius tadpoles and now several FG Vent tadpoles in this way ---

I take a one gallon bottle of Crystal Geyser spring water and pour a little bit out and into another container. I then take half of and Indian Almond leaf and cut it up a couple of times and stick it in there. I ocassionally shake the bottle. After a day the water starts turning light brown. I start using the tea at this point.

Now here is where some people might disagree with me. Whatever container I use for the tadpole the lid doesn't have any holes. In the tadpole tea I put a small piece of Indian Almond leave and some Java Moss. Every couple of days I will put a tadpole bite laced with Sera Micron. I also put in a couple flakes of OSI Spirulina Flakes. Two days later I change the water and I start again. I have never lost a tadpole.

If I had a Pumilio or an expensive or delicate thumbnail tadpole I might do things differently. Maybe.

I live in California and sometimes I forget that people live in other states that get really cold when I'm having 70 degree weather. I have never done the heated water bath thing but I do put the containers inside a larger Rubbermaid or Sterilite container with water on the bottom instead. This is just to bring up the heat and humidity a little.