Exos are very very fast fish, they can pretty much out swim anything in an aquarium, but the trick is, is that when you intoduce them to a tank with pirahnas the pirahnas know the layout of the tank and are used to the water in the tank. So to overcome this and increase survival of your exos get them aclimated to the water (the way I do it is a tank in tank, I have a small 5gal that I clamp to the main tank and put a powerhead with a hose into the 5 gal--> water from the main tank goes into the 5gal and overflows back into the main) I leave them there for a couple of days. Then I move everything around in the main tank so the P's dont know where they are, release the exos, and tada--P's and Exos swimming around.