A sump is basically a place for water to leave your tank by gravity (via a 'downflow box', either built-in or hang-on-back [HOB]) and be cleaned, skimmed, aerated, heated, and pumped back up via a 'return pump,' aka, your main pump. This gets all the unsightly gear hidden under your stand.
This is why wedge tanks (which I have) are a pita to fit a sump into. I took the other route, ran a downflow hose and a return hose through my floor (or your wall, above the baseboard) down to a basement sump where I have a floor drain and all the room I need.
They also provide a place for unwanted crabs to live a useful life, a place for dead coral bits to lie and be biologically useful until you need to glue a bit of live coral to them; and a place you can add things like calcium and buffer without dumping it in where the fishes live: the return pump mixes most things nicely. If you light it, you can put macro down there (cheatomorpha algae is good) and raise pods for obligate exotic fishes (like mandarins)---
It's also 20-30 extra gallons in your system, and the more gallons the more stable your system is, chemically. They don't have to be fancy: a poly bucket can work for a small tank. When you get up into the large tank, you'll want a larger sump: large is about 30 gallons. But it's also about how much room you have to put it in.
Because it requires that downflow box, it's a need well to think of in the planning stage of a tank, because HOBs are a little bit of a pita, and built-ins are rarely any trouble. Don't worry that your tank will drain down and flood: proper downflow boxes have a tricky inner wall that dictates how much water CAN drain. Getting your system water set often requires a friend with a cellphone (WHoa! cut the pump! [as you dip out more water]) but it's not rocket science.
I do recommend them over the contained canisters for safety. Your sump will NOT overflow. My personal experience with a pretty pricey canister was not so good on my living room floor. Seal gave way. And they CAN just go on when they leak. A regular doublewalled downflow box or a hob with a siphon would have reached the cutoff point and stopped. Just sayin'. Based on my experience, if you have a canister---set the thing in a Rubbermaid bin.
This is why wedge tanks (which I have) are a pita to fit a sump into. I took the other route, ran a downflow hose and a return hose through my floor (or your wall, above the baseboard) down to a basement sump where I have a floor drain and all the room I need.
They also provide a place for unwanted crabs to live a useful life, a place for dead coral bits to lie and be biologically useful until you need to glue a bit of live coral to them; and a place you can add things like calcium and buffer without dumping it in where the fishes live: the return pump mixes most things nicely. If you light it, you can put macro down there (cheatomorpha algae is good) and raise pods for obligate exotic fishes (like mandarins)---
It's also 20-30 extra gallons in your system, and the more gallons the more stable your system is, chemically. They don't have to be fancy: a poly bucket can work for a small tank. When you get up into the large tank, you'll want a larger sump: large is about 30 gallons. But it's also about how much room you have to put it in.
Because it requires that downflow box, it's a need well to think of in the planning stage of a tank, because HOBs are a little bit of a pita, and built-ins are rarely any trouble. Don't worry that your tank will drain down and flood: proper downflow boxes have a tricky inner wall that dictates how much water CAN drain. Getting your system water set often requires a friend with a cellphone (WHoa! cut the pump! [as you dip out more water]) but it's not rocket science.
I do recommend them over the contained canisters for safety. Your sump will NOT overflow. My personal experience with a pretty pricey canister was not so good on my living room floor. Seal gave way. And they CAN just go on when they leak. A regular doublewalled downflow box or a hob with a siphon would have reached the cutoff point and stopped. Just sayin'. Based on my experience, if you have a canister---set the thing in a Rubbermaid bin.
