My new setup is 400 gallons. It is drilled for four 1 1/2" drains. I wanted to take two of the 1 1/2" drains and combine them into 1 2" drain. So when all is said and done the four 1 1/2" drains will choke down to two 2" lines to my filter system. I used the drain calculator on another site (thanks dan) and so far everything I researched seems to be cool with doing that. I'm trying to stay on budget and not have to buy a bunch of 3" PVC and fittings for my drains. keep the input coming I'd appreciate it.