Re: hatchetfish.
Marble hatchets and black-winged hatchets tend to stay very close to the surface. They grow to about 3-4 cm and should be kept in fair sized groups (6 or more) because they can be rather nervous.
Common hatchets, the type with the single black stripe along the flanks, is a medium sized species that gets to about 6 cm long. The "giants" are things like the silver hatchet (which has a single blue stripe) and the spotted hatchet get to about 10 cm long. These larger species are fairly robust animals provided they are not kept in very hard, alkaline water.
I keep my common hatchets in soft and acid water, and they're among the most outgoing fish I have. Very aggressive feeders, they happily take food from my fingers, and will even nip me if they get too excited. The larger hatchets will swim about 10 to 20 cm below the water level a lot of the time. They aren't particularly nervous, and my group of 4 seems perfectly happy. They sometimes chase one another but no harm seems to be done. They get on very well with glassfish, oddly enough, and the two species form loose schools together much of the time. Highly recommended fish.
Cheers,
Neale