5 white clouds
a pair of rhinogobuis wui (not that colorful but cool looking.
I would say depends partially on how cold/warm it is where you are.. but zebra danios are often kept without heaters just fine.. at least they are in the fish labs in the building I work in, they must have over 6000 danios in un-heated tanks..
I'm liking the gobys...not sure where I would find any around here.