I think a monster tank is a tank with monsters in it. For example.. my tank is a mere 55g, but it has a very grumpy Jack Dempsey living in it. He is mos def a monster.
I'm hoping to set up a 96x48x30 in the next couple of months. I will be buying some sheets of acrylic and doing a DIY project. The Milkman makes it look awful easy. I would consider that a monster fish tank!