The first thing I'd suggest is to splurge 3 bucks to buy credits, so you can trade freely on the game. Also, be sure to buy Danger packs, 'cause that's where the Piranas are at.
Since yer lvl 16 and probly don't have a lot of clintz, I'm going to tell you the best inexpensive Pirana cards, IMO. Please note that I will mark my favorites with a .
From there on out, they get more expensive. When you get the dough, you're gonna want
Smokey and Hawkins (though Scubb can be fun to play with).
and either Bloodh OR Dahlia (you don't necessarily need both. Depends on your playing style. Dahlia's pwr is awesome, but Bloodh's dmg is intimidating).
In my experience, Piranas don't usually make a good mono clan. In other words, they work better in a half deck (half Piranas, half another clan). They pair well with Skeelz and atk manipulation clans, IMO.