Most of the worm experts I have talked to here say it is not necessary to do the after 10 day treatment, following the 3-5 day treatment with fenbendazole. I'm not sure why, but you could still do it if you wish. Normally, it is to get the hatching worm eggs after 10 days. There is not a lot of studies published on using wormers and re-treating. Contact Casportpony if you wish--Kathy usually has the most up to date info, and she usually does her own research on her poultry as well.
I would rule out a respiratory disease if you think it is gapeworm, since gaping can sometimes be one of the common symptoms such as watery eyes, nasal drainage, rattles or wheezes when breathing, etc.