Zebra finch with a messy beak. Could this be a crop infection? And how can I treat it?

Q: I have a zebra hen that has for a few months  presented with a messy beak , she is lively and  eats well, though her water feeder is always full of husks? I have cleaned the beak a number of times only for it to reappear again. There have been times when she has looked as if she is going light but always recovered. Her condition is okay but not good, her plumage is a bit ragged so to speak. I have been told it could be a crop infection? and to give her apple cider vinegar, which did appear to work for a while only for it to return again. No other birds in my shed have the same symptoms , Any ideas please??

“According to the symptoms, we think it is probably mega-bacteria of the crop or possibly a fungal infection in the crop. By giving apple cider vinegar the higher acidity of the water probably helped for a bit but seems it was not sufficient. In this case, we suggest use of Nuovo GR at the standard dose (1 mL/liter of drinking water for 2 weeks) — this product has both anti-fungal properties as well as immune-stimulant properties. Then wait one week and repeat another 2 week cycle at 1 mL/liter. During this entire period, give Biointegra daily (100 grams per Kg of feed); then continue use for at least another 1-2 months. Biointegra is important with this problem as an efficient intestinal microflora is of great help with both mycosis and mega-bacterium. 
As a note, crop infections due to mega-bacterium or mycosis typically result from use of antibiotics which deplete the intestinal microflora, leaving easy access for mega-bacterium and fungal infections.”