Scissortails are super skinny fish outside of their enlarged fins so they don't take up much room. One thing I will say is that you definitly need a tight fitting lid with no gaps on both the QT and DT. They are unbelievably quick and easily spooked. I had one in my 36 gallon and it could vanish in the blink of an eye. Mine also attempted to jump out a couple of times and hit the hood hard enough to make a loud sound. They should do better as a trio. I just hope yours don't spend their time in hiding like mine did. Between my experiences with my Purple Firefish and my Scissortail, I have sworn off Dartfish for good. Way too easily bullied by other fish and spent too much time hiding for my tastes. *EDIT* Here was mine when I had it: