If we assume that fish weigh their volume of water (as the fish can move up/down in the water column, it's not very difficult), we can roughly estimate their weight by calulating the volume of the fish... One cubic centimeter (cm3) of water is one gram.
For example, a cory is more or less a triangular pyramid. So V = (1/3)(W * L * H). So if your fish (not counting fins) is 3cm long, 1cm tall and 1cm wide, it would have a volume of (1/3)(1 * 3 * 1) -> 1/3 * 3 = 1 cubic centimeter -> 1g.
A large cory, such as C. paleatus, grows to 7cm length (not counting tail-fin) and perhaps 1.5cm tall/wide: (1/3)(1.5*7*1.5) -> 5.25 cm3 -> 5.25g.
As you can see, the weight of the fish goes up quite dramatically as it grows, because it grows in all three dimensions...
Obviously, not all fish are pyramid shaped, so it may be better to calculate them as a cylinder or prism ("box").
Here's a set of formulas for volume of a few different:
http://library.thinkquest.org/20991/geo/solids.html
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Mats
