Tom

I have used Rhodamine-B (hot pink) for several Petersen mark-recapture
sampling projects.

The hot pink color is so intense that miss identification is virtually
impossible.  I tested Rhodamine-B, Alcian blue, and Bismark brown side
by side at various concentrations and found the Rhodamine was by far
the visible dye.  It was so obvious that you could identify dyed fry from
10 feet.

I found that Rhodamine-B @ concentration of 1:10,000 (1 gram to 2.5
gallons of water) was very effective.  Placing fish is an aerated holding
tank for twenty minutes was good enough for the dye to last several
days.