Thanks for your advice so helpful.if you don't really know nothing bout it please don't reply thanks!
If the problem is due to the cresting, then they're right, there IS nothing that could be done.
Cresting is caused by a fatal gene. Any duck that gets two copies dies before it can hatch. The crest is caused by a hole in the skull, which the crest sits on top of.
Additionally, the gene can cause all kinds of neurological issues, like seizures, walking in circles - and lameness.
But before jumping to the conclusion that this definitely the case, the niacin issue should be addressed first.