If you raise fish, you eventually have to move the fry from one place to another. Catching fry is not always an easy task, especially for those fry that are unusually quick, particularly small, or extremely sensitive to shock due to handling or different water.
One of the simplest and safest methods for moving such fish has proven to be the siphon technique.
First remove any plants or decorations from the container to be emptied. Then, using clear plastic tubing of sufficient size, siphon off the majority of the water. Most fry will stay well clear of the intake tube and the fast moving water, but be certain the discharge goes into a holding container so you can retrieve any fry that get sucked up mistake.
Now, using the siphon itself, a turkey baster or a large pipette/eyedropper (for fry), or a plastic container and a net (for adults) collect the fish that remain and transfer them to a holding container. For small fish clear plastic 1 lb. food containers work well. Float the container in the new tank and acclimate in the usual manner. You have already, of course, made certain that the water parameters and temperature of the tanks are similar, to minimize potential trauma to the fish.
The siphon technique may take patience and practice, and it may result in drippage or spillage, but moving fish in water is much better for the fish, and travel through a siphon tube or turkey baster seems to cause few if any ill effects to fry. As fry grow, they seem steadily less likely to suffer damage from handling, and by the time they have reached 1/4″ or so, most seem able to manage being gently caught using a net.
— GCKA Newsletter, August 1998