Piranha Attack!

So my neighbor Daniel asked me to feed his Piranha, while he was away on a business trip. The aquarium which houses said piranha is also home to 7 goldfish that swim happily (or, more likely, nervously) around. The Piranha is supposed to be fed a piece of fish fillet every 3rd day, which I've been doing. The instructions are you tie the fish fillet to a piece of string and dangle it into the water, moving it around to simulate a live fish, I guess.

Except yesterday, it just refused to eat. I tried for about 20 minutes with no success. Here are pictures (the water's very murky):




Anyways, I show up this morning to try again and I'm met with this:


He'd torn one of the poor goldfishes to pieces! What do I do?