That doesn't look like any powerhead I have seen. It looks like it is specially made for some specific usage. Can you remove the pump rotor to see what it is like? For a 15 gallon tank you don't need a big powerhead for CO2. Just a tiny little one like the Hagen Elite would be more than big enough.
Don't try to use this as is, or it can easily suck the DIY solutions into the tank. But, if you can remover the small diameter tubing and connectors leaving only the large diameter inlet to the pump rotor, you may find a way to make it work.