The 3x5 Photo Tank holds fish (like small sunfish) which are too large for the Micro Photo Box. It also holds several micros at once, which makes it much easier to compare several similar looking fish. Also, with more than one fish in the tank, they seem to settle down more quickly so you can get better photos. Molded from a single piece of plexiglass, it has no seams to leak. The hinged lid will keep fish from jumping out. Inside dimensions roughly 3.5" x 5.5" (3" front to back).