I'll give the win to you daizeUK, but just for anyone interested the best way to measure a fish is with a solid ruler that has a stop at the end where the fishes mouth can be butted up against and then the body laid straight along the ruler. For the health of the fish when measuring like this it is also best to wet the ruler and have your hands wet as well.Another measuring option is as Mamashack said holding a ruler against the glass and trying to measure the fish as it swims past but this is going to only give you a rough length of the fish.For guidelines on how to measure crabs see Crabs and lobsters .Line drawing of fish measuring board showing how to measure fishMeasuring device. Fisheries Queensland recommends using a sturdy ruler or coil tape measure to determine the length of catch. Adhesive or stick-on devices can shrink when exposed to weather and be unreliable.