It is difficult for me to eyeball any of my fish for a good measurement. I read about fish sizes and all a good bit on the forums and some are hard to believe, some not. I thought my male istlanum was 5 inches until I moved him recently. I broke out the tape and he was 6.5 inches! So I tend to underestimate. I don't take fish out of the tank just to measure them but if I'm taking one out anyway I'll put a tape up to him in a minute! I've heard of rulers up against tanks and such and I have even put gradations on the side of my tank with a marker to see how big the fish is when he lines up with it. Can't really see the gradations that well unless you know what you are looking for and they can be placed anywhere on the tank. Wipe off pretty easily too. Anyone else have any methods they use to measure fish?