Oxygen permeability.
I'm unsure.
Latex is _damn_ thin.
if it is not lubricated, you have a really thin membrane, that's got quite a large surface area.

www.ncbi.nlm.nih.gov/pubmed/1559333 says that 240ml of water filling a condom makes the thickness is .04mm or so.

If you fill it to a couple of litres, then that will drop to .01mm or so.

www.biolbull.org/cgi/reprint/94/1/66.pdf Gives the metabolic rate of goldfish at 25C as around 80cc/Kg/hr.
Or for a 3g goldfish, around 1/4cc/hour.

I can't seem to find data for oxygen permeability of a .01mm thick latex membrane .01m^2.

However, if you fill a condom to 2l, will it deflate to 1l in under 4000 hours - or around 6 months?

This is not seemingly completely unreasonable that it might be lots more permeable than that.

Unlubricated would probably work better, and spermicide might be toxic to the fish.
I have not contributed to science as yet by performing the experiment.
(speedevil, Tue 3 Aug 2010, 18:27, Reply)