55-100? What would the dimensions of the bottom be? I have a regular 55 gallon tank (I just looked back and see you said something about a 55 gallon long), and the distance from the front to back glass is just over 12 inches. I would personally not put a 12 inch fish in a tank with a 12 inch area for turning around. That is too tight, and cruel, especially since they will appreicate decorations and/or plants for playing around in and hiding, but they would get in their way as they tried to move. I was under the impression they were already in a large tank (don't ask me why), but I guess not. I'm not sure how fast the tiger loaches grow, but if you have them in a tank that is too crowded or too small, they may become stunted and not reach their full potential. At least, I know that happens with clowns. Best to get the larger tank as soon as you can. 100 gallons sounds about right. If they would do well in less, I'd have one, too!