Would all depend on the size of the pirana. If it is the size of a quarter then will probually be to much. But if it is a couple inches or bigger will be fine. I use ac110's on 40 gallon breeders all the time. I have run 2 ac110s on standard 55s a few different times.
far too little especially if its a larger serra bc they have large waist loads or a small pygo bc they grow fast and eat a lot i have a ac110 and a fluval 300 for my 6 inch diamond pluss my tanks heavily planted. the rhoms are wild caught and stress super easy under poor water conditions