Taken from http://en.wikipedia.org/wiki/Guppy

"The guppy prefers a hard water aquarium and can withstand levels of salinity up to 150% that of normal sea water,[9] which has led to them being occasionally included in marine tropical community tanks, as well as in freshwater tropical tanks. Guppies are generally peaceful, though nipping behaviour is sometimes exhibited between male guppies or towards other top swimmers like platys and swordtails and occasionally other fish with prominent fins such as angelfish. Its most famous characteristic is its propensity for breeding, and it can breed in both fresh water and marine aquariums."

This is so interesting! Who would have thought a guppy could withstand a marine environment!! They are tough guys!!!!