Nope, are too active and grow too big for a 2.5gal- need somthing more in the region of 15gals as they are schooling fish and need to be in groups of at least 6 as far as im aware . They definatly need a filtered and heated tank as well :nods: .
Tokis is correct in that they do need a bigger tank, but a trio (2 female : 1 male) would be perfectly happy together.
I've just done a quick google search on minimum tank sizes and the opinions seem to vary from 5g to 30g! That's quite a big difference.
Although they are active, in my experience they are not zoomers like danios are and tend to be shy under bright lights. I would therefore think that a 5g planted tank would be okay for only a trio, but I'm sure that someone will correct me soon!