Ummm, i think that tank is alittle overstocked, 5 guppies in a 2.5 gallon tank is not a good idea at all. They grow too big, and produce too much poop and babies.
Her injured fin is most likely a result of fighting between females, and too little space.
I would suggest you take the guppies back to the store you got them from and get a betta . Or get atleast a 10 gallon tank instead.
In a 2.5 gallon tank you could have a betta, possibly with 2 julli cories or an apple snail. Nothing else is really suitable.
Sorry, hope it helps.
(If the guppy still has a ripped fin add about 1tsp of AQUARIUM salt to the tank per 5 gallons).
P.S Colin...2 bala sharks in a 15 gallon doesn't sound like a good idea either. they grow too big. And bettas aren't normally too aggressive, although it depends on the indiviual, normally you don't keep bettas with guppies or fin nippers or gouramis.