Hi mako welcome to Fish Lore
Can I suggest you move the betta in the 10g? In order to thrive, they need a heated filtered tank.
Sharks need much more room than a 10g. Most small fish are schoolers, so there's not much variety you can get to fit in a tank that small.
Please read about the nitrogen cycle:
http://www.fishlore.com/NitrogenCycle.htm
Cycle without fish, you'll be doing yourself a big favor and no fish lives will be lost.
Read up on any fish you decide to get, make sure you know their tank requirements and compatablity.
Good luck.