I built a basement sump & fuge setup last year and found that with a lot of planning, it can be rather painless with tremendous benefits. Here's a few things I learned:
1- going through the floor is simpler than the wall - easier to cut and fewer resultant bends in plumbing because you can go straight down. I have hardwood floors and cut rectangle that I can replace in the future and refinish like new.
2- Hard PVC plumbing is a more reliable way to go with fewer connections to worry about
3- A big sump is good, but more volume means more additives and more food like phytoplankton. And definitely requires a bigger skimmer.
4- Stock tanks are an excellent choice for cost and convenience reasons. The bigger you go, the less you need baffles - I have a 150g and no problems with return bubbles. Bulkheads for hard PVC are easy to install if you don't use the standard hole. You can rig up strained PVC returns for pump feeds inside the sump to avoid sand and junk from getting into your expensive pumps.
5- A basement sump means skimmers and pumps can be OUTSIDE of your sump- and that makes everything easier
6- Make sure your drain pipes do not go too far below the sump water surface level or else backpressure will reduce return volume
7- Salt spray will corrode evertying within 20 feet of your sump - an issue if you have furniture or metal equipment stored in your basement. Wrapping a drain pipe outlet with a skirt made from plastic fish bags is an almost perfect solution. Rubberbands can hold skirts in place. Total solution would be an acutual fish room with positive pressure ventilation.
8- Hardware stores sell basement water sensors for $10. They are good for under the tank too
9- Make sure if you put ballasts under the floor that you have drip loops and that you thought about all water routes in case your upstairs tank fails. Saltwater and ballasts - no good!
10- Think more about in-tank circulation than volume up and down the floors. It costs less and gives the same result.
Okay one more - 11- if your heaters are all in the basement, consider what will happen if your main pumps fail and your display gets no heated water supply. I haven't come up with a satisfactory answer to this yet.
Okay, last one - I mean it - 12- My basement stays very cool year round. In Winter, to be safe, I run 5 watts per gallon. 1250 watts total.
