eye

I transfered my bloodfin tetras to my new 36 gallon and they have been in there for a day. This morning when I went to feed them I noticed one of the has a bulging eye. It is bulgling out all the other fish are fine. It's just the ones eye that is wrong anyone know what this is?

I feel like I've already posted rather a lot of threads on this site, but I've found it so helpful and I'm quite worried so here I go, posting another one :-P I have a 20 liter tank which I'm currently in the process of moving to a 60 liter tank. well I haven't really started doing this yet...