I've had a red turq hanging out in a corner of the tank and not eating for about a week or two now.

Last night i witnessed a white poo hanging from his vent. It didn't appear "mucusy" but more normally formed, but i'm not going let that fool me.

Lately it's been comming back out and being social, but i haven't positively identified it eating.

Anyway, i've been doing a bit of searching around and there is fairly strong evidence to suggest that he may have "hole in the head".

So, i have a few questions:
-Do i treat the whole tank with Metro? Is this a diease that is likely to be contaminating the whole tank. I have a handful of tetras, bristlenose (peppermint), and corry's in with the discus in that tank. Will this worry them?
-I have another tank, and i use the same siphon and pump hoses for both tanks at WC. Is there a chance that cross-contamination may have occurred? Do i need to treat both tanks?
- I have to go way for work for a day or so this week, which will interupt the ideal WC and retreatment cycle of the metro. Will this have a really detrimental effect on the treatment process?

How would this have got into my tank/fish. I do 30%WC twice a week with aged water, and run UV steriliser also. What am i doing wrong.

The fish in question is the one in my avatar. Picture was taken on Friday last week. I know it's too small to see anything really, but the actual larger image is on my home PC and i'm at work atm.