I recently introduced four female and two male danios to my well-matured 30gal tank, which also contains a small shubunkin, fantail and a moor. all was going well until two days ago, when I noticed that only 5 danios were schooling - closer inspection revealed the sixth hovering listlessy behind a rock... with one eye missing o_o

needless to say I was shocked and worried :/ he picked up yesterday, schooled for a while and took some food, but again today he's hiding and not looking too good. he's staying upright and swimming a little but making no effort to keep up with the others.

could it be that the other male got a little aggressive with him? they seemed fine for the first week but of course I don't know what they get up to at night, though my trusted fishseller assured me that two females per male was a good ratio to avoid aggressive behaviour.

what treatment should I give, if any? would isolating him be a good idea? (I have a cycled 10gal that I was planning to put a betta in that could be an isolation tank if necessary). what are his prospects for survival with only one eye?

my boyfriend renamed him Cyclops

My bet would be the shubunkin got him.
Goldfish see smaller fish as food and if you dont take them out then they will all get eaten or be damaged in some way.
The one with only an eye is now handicapped and will not see a hungry goldfish on his blind side.

Yes, I agree with Sandy. I keep white clouds in with my bubble eyes, but they couldn't catch them if they got help Shubunkins are a different story. Very fast and agile.

I have a fish that recently lost an eye (and almost lost his last remaining one too!!). I made sure the water was perfect and added Melafix to prevent any infection. It worked a treat!

Make sure the water param's are perfect! You want to make sure the lil' guys immune system is tough!

the shubunkin is the most placid and peaceful fish in my tank

the water params were spot on in my spare tank so moved him into there, with a little encouragement he's taken some bloodworms and is picking up.

I am glad to hear he is improving