just got a new 120 gal tank

I bought a 120 gal tank from Pets Mart and was curious on what filter system i should get. There will only be 3 piranha in there for a while and they are only 3 inches at the moment. I have a friend that is going to let me borrow a whisper 5 for a while and i am pretty sure that do till i get more piranhas.When that happenes what should i get?

The tank came with an under gravel filter should i evern bother with is to clean the accumulation of poo in the gravel? The gravel i am going to use is a little bit smaller than pea gravel, will that be ok cause it is free?

What should i do to get the water in the 120 gal ok for the piranhas? Should i take some of the water out of the tank the P's are in now and put it in there with treated water?

And what kind of fish should i put in the large tank to get the water ok for the P's. One of my P's has something wrong with him so should i do something special to teh water in the big tank?

sorry bout so many questions......i am fairly new at this adn all the books at the library just had info on the fish.