just to add, make sure your filter is going at the rate of 10x per hour of the tank size. So 120 gallon tank should have at least 1000 GPH filteration/hr if you are planning on keeping a full stock of red bellies in there (recommended by most piranha keepers at piranha-fury.com). like the other member said earlier in the post, you can probably start with 8-10, they will eventually trim down to 6 or 7 adults.
Also if you want to make them more comfortable, adding driftwoods wouldnt hurt. The woods will give out a slight brown tint to imitate the amazon. Make them more comfortable to the environment.
As far as your UV sterilizer filter question, you can add a canister filter and put the UV sterilizer on the return hose of the filter. This way cleaner filtered water will go through UV light case instead of dirty water. Keep in mind that your UV light will have to be replaced periodically.