I don't have a lot of free time at the moment or I would chase down the link to confirm my memory (for those interested I suggest the frognet archives). If I remember correctly, Dendrocare was formulated with preformed vitamin A but the amount of preformed vitamin A was reduced as there was concern that preformed vitamin A was causing issues with the frogs.
We know today that this is not true unless one strays from the proper ratios or really overloads the frogs.
A phrase you never want to hear;
"It seemed like a good idea at the time."