The only issues I see is the bottom tank is always going to get dirty water form the upper tanks, and I think a mag 5 is alittle weak for 100g. But this is a different setup, If you have the pump already I would try it atleast to get an idea of the flow needed. I have a mag 7 for a 45g, and I like the flow. So a mag 5 on 2-55g, I think it will be weak. Just my 2 cents, Stephen.