I can't remember what size tank you have, but (and I'm sure others on here will give you more knowlegeable advice) I think more than 1 is only a good idea in a tank > 40 (imperial) gallons.
I made a mistake and bought 2, their tank is too small even though they are only 1.25" each. The tank is 24x15x12. They are ok at the moment but I need to rehouse them ASAP.
I would keep the one you've got and learn how to look after him / her as best you can.
The 'going up and down the side of the tank' thing is more to do with a lack of space / territory I think (think of a tiger in a cage) so adding more won't help.
Ade.