Place a jar in the tank--best if it's not clear, or can be covered to create a shadow. If the jar is in the tank for a few days, you have the option of 'baiting' it with a preferred food. Eventually, the fish get used to the jar, and will happily swim into it to hide. Then it's just a matter of putting a net over the opening, and lifting the whole thing out. Guess this will be harder in smaller tanks, but it works great in medium sized tanks, and with fish that have lots of spines that can get stuck in nets, or for fish that panic if removed from the water.