If I had a 37g, give or take, at my disposal with those dimensions that you specified. I see no reason why you couldn't opt for something a little different and have a corydora species tank. There are some species, especially the pygmy corydora that cruise all parts of the tank. In addition, in a large school, you really get to see some very interesting behavior.
Say, go for 15 corydora pygmaeus
8 of a more medium bodied corydora with an interesting pattern. Green lazers are really lovely.
8-10 of albino corydoras for some contrast
Another option that would be really great, if you want a species tank, are tiger barbs. They are a fish that really come into their own in a species setting. You could get the different varieties for contrast (green, albino, etc), but I really like your old-fashioned tiger barbs. The males are particularly impressive when they color up.
Some of the more unusual tetras are also an option. I once kept a species tank of lemon tetras. Really very lovely and they had the most subtle colors. Quite beautiful in a planted tank.
Finally, an option, especially if the tank is as long as you say it is, like 48", are some small danios. They would really enjoy the open spaces in your tank, and you could really get a nice school of say 20 zebra danios, leopard, or even pearl danios (gorgeous colors when in the right light). Compliment them with a few of the smaller loach species, like botia striata or kuhlie loaches, since you have sand, and that would be a kick butt setup.
That, clareuk, is what I would do with your tank if it was mine. Hope this helps a little. Keep us posted on what you decide to do.
