A sump is a smaller aquarium usually housed under the main aquarium. Water circulates from the main aquarium into the sump and back again. The real advantage is that you can put various pumps and filters and your protein skimmer down there, and not have them in the tank itself or hanging off the side.
Right, both of you! 50's are just about the same price anyway, and they look much better...
If you are going to use a sump (HEAVILY recommended) you may want to ask your dealer about drilling the tank...makes the tank top securing situation much easier!
I'm thinking about getting a 50-70 gal tank, and I've had my eye on the Fluval 404. I'd do a sump, but one question is bugging me: what happens in a power failure? What's to stop the entire tank from overflowing down into the sump?
Good question jetflee, but that is not a problem if you do it right. The idea of an overflow, whether a box or a drilled hole in the main tank, is that if the sump's pump fails, water will drain out of the tank until the level goes below the overflow. Then, on the sump end, the tank is usually only 1/3 or 1/2 full, leaving plenty of space for the extra water to fill in from the main tank in case of a power failure. This is probably the most genius idea in the hobby, besides the protein skimmer and refugium filters.
i have a 55g that is almost ready for my bimac, instead of a sump i got a amiracle hang on the back wet/dry filter with a built in skimer, it has a few advantages, first about the power fail, the water is pumped into the filter via a small pump in the tank therefore if power fails water stops leaving the tank, the returns are enclosed L shaped tubes which makes it easy to secure them, i put a sponge in them, also the pump goes directly into the skimer so every drop of water goes through the skimer first.