I have a dwarf in a 3 gallon with a beta, he dominates the bottom and the betta dominates the top (predators typically leave each other alone, but this is not always the case tho make sure your the betta is dosile). HOWEVER the dwarf cpo is a fragile lil guy he needs tons of food, hiding space (to molt), live plants (to eat), lots of moss (to eat), and exceptionally clean water at 200 tds. I have sucessfully kept mine for several months in such a small environment, but its no jarrarium. The water runs through 3 layers of charcoal filtering to keep him happy, annnnd I have the tank heater to 78 degrees. There great pets to own, but Im nearly certain he would die in a filterless ambiently heated environment
https://www.amazon.com/dp/B00891I1Q2/ref=cm_sw_r_cp_apa_4anPzbTSF1SGW
This is the smallest tank I've managed to keep one alive in with a bit of tweaking on the filter. This is a fairly inxpensive rig, it's pretty low tech in terms of lighting. So I advise keeping it near a window for cheap yet effective light.
So in conclusion i highly advise raising one of these guys there a riot to watch (so cute :3), but if your hearts set on a jarrarium use either neo shrimp or snails. If you can get a filter in the jar potentially fish.
https://www.amazon.com/Marina-360-Degree-Aquarium-Starter-Kit/dp/B00891I1Q2
The heater was sold separately.
Warning: It's acrylic plastic, so under little to no pressure, small almost scratch like lines form inside the plastic pane, which may ruin the experience for some people.
What kind of tank is it? It looks kind of similar to something I have in a box, and I want to make sure it's not as I was thinking of putting a betta in there. I have something like, if not exactly, this: http://www.amazon.com/Hagen-Marina-360-Degree-Aquarium-Starter/dp/B00891I1Q2/ref=sr_1_2?ie=UTF8&qid=1420948922&sr=8-2&keywords=2.65+gallon+fish+tank
Well if you want a fancy looking aquarium with hidden equipment the Marina 360 is 2.7 gallons and only $36. A bit pricey, but a lot less than Fluval.
Marina and Fluval are both owned by Hagen though, so it should be good quality^[1] (haven't set mine up yet though).
^[1] Link