there is not need for that petromalt for most people if they are feeding a balanced diet.
i honestly dont think you should give it to them. i have heard of it keeping vitamins from absorbing after prolong use, and i have heard about it actually making things worse by causing "clogs"
During a heavy molt, or the first signs of not pooping much i give my bunnies wild grass then they will pass everything up like a storm. however i have never really dealt with a heavy mold. Yesterday Ryo ate some rubber of a shoe, i gave her some wild grass to help her pass it. i am not saying that it actually works, but it seems to work for me.