Under gravel filter cycling and parts?

So I got this new tank today and I'm just assuming that this thing on the bottom is a under gravel (?) Filter.. I've never used any filter besides a HOB filter so I need all the info I can get (including all the parts) If its broken/ missing parts then its not a huge loss, I got it at a thrift store for $4
If it doesn't work out, I have a 5 gallon HOB filter
Its a 2.5g tank.. I think.. I'll post a pic in a sec.
My other question is, is the Nitrogen cycle the same for this kind of filter? If not how do I cycle this tank?

 

It came with a bubbler, if that's what the plastic tube is for
Also, how do I tell what size the tank is?

 

From what I know, UGFs used to be popular but have fallen out of favor now. They aren't customizable like HOBs, offer only biological filtration, and are a pain to clean.

Go with the HOB!

EDIT: Not sure how to calculate the size of a hex tank, maybe just add a gallon of water at a time until it's full?

 

Ok, its just that it came with it (if that what it is, I don't even know.. ) So I figured I would try it
I don't feel like being challenged right now so I guess I'll stick with he HOB I have

 

Not that I don't like your suggestion (its probably the best imma get)
But does anyone else know how to calculate the size of a tank?

 

Ooh, I actually found this calculator:

Scroll to the last one, "Multi-sided (Hexagonal) Aquarium".

 

Thanks, figured out its about 2g