What?! You don't have a fish scale????
j/k
That's a good question, we actually had to weigh fish that died in the QT process before treating them. Small fish were put in a beaker of water and we measured the volume of the fish by how much water they displaced. Usually before the necropsy to determine what they needed to be treated with (ie. we would collect a small school of fish from the same area and they would all go through QT at the same time in the same tank if possible).
There are general formulas for estimating fish weight by length and girth, the formula differes depending on fish type/shape, and generally is based on pounds, but can be converted to grams for smallish fish. Unfortunately the room for error becomes large at the small size (the difference between a gram in a fish this small is bigger than in a 10 pound bass).
You could either net the fish and put him in a small beaker or graduated cylinder with SW and measure how much water it displaces (dunno what you have avaiable I don't have any graduated cylinders in my house
)
The only equation I have for estimating a fish's weight by measurement is:
((Girth x Girth) x Length) / 800 = weight in pounds
then convert pound to grams via:
pounds x 453.59237= grams
HTH