Some of my young koi are losing their scales. I am already treating them for both bacteria and parasites. They don't seem to be having any problems with it but do the scales grow back?

Fish lose scales all the time, usually due to scrapes and other contact. If the underlying skin is not broken and the root of the scale is not destroyed (as it would be with an ulcer) the scale will grow back as the fish matures.

Try not to overtreat your fish. Overly aggressive use of antibiotics will encourage the growth of resistant bacteria and you'll end up with untreatable infections. Water quality is the key. Filter, filter, filter!

(Answer courtesy of Bob Passovoy).