I've keept the common SD's and spotted SD's (smaller) in all of the following: 40b, 55, 60, 110, 150, 240 gallons. In my experience they dart any way, front, back, side, and even into the ground or right into the lid of the tank. Even in my 240 they run into the front and back of the tank. That's just their nature. Length of the tank can be addressed but width will still be a problem in any standard size tank. 40b has the width advantage over a standard 55 and 60 gallon. A 100/110/125 will still have the same width as the 40b, which will be your biggest problem with them running into the glass. Going to the 2ft width helps but they still run into the glass. Ultimately the most important thing is getting a group to make them feel more comfortable.