Hows about just giving the ruler a good clean and put it next to them in the tank? Simples! I just open the lid and my guys swim to the surface to beg for food, they will float there untill they get their worms so I just measure with a ruler from above. Gomez never swam to the top and floated about like the others so the ruler just went in the tank to measure him, prob is he's a lot longer than any ruler I can find now so havnt bothered in a while! The ruler against the glass is a good rough guide tho, water and glass can certainly distort things abit tho my axies were an inch bigger when measured with a ruler next to them instead of through the glass!