sump is basically just a low reservoir that collects water as it drains from something above it. When you hear the term used in an aquarium, it's referring to a reservoir that sits below the main display tank. Water is pumped up from the sump into the tank, and then drains via an overflow back into the sump. In a way, you can think of a sump as a something like a huge, flexible, and accessible canister filter.
There are many advantages to this kind of setup:
- You can put your heater, filtration, and other life-support equipment in the sump, rather than having them in the tank. This makes it easier to service them, and hides them from view. It's also a convenient place to dose any additives you use, or take water for water quality testing.
- It's often effective to build the mechanical and biological filtration into the sump, because you have so much volume to work with. Sumps are typically much, much larger than any canister filter you can buy at a pet store, so even using a portion of one this way gives you a relatively huge amount of filtration. This is often why people use a sump in the first place.
- It increases the total volume of water in the system, which helps keep your water quality stable.
- Because the sump is actively pumping the water in it up into the tank, any water that leaves the system is effectively draining the sump instead of the tank. This means that your tank's water level won't drop from evaporation, and it allows you to do water changes without disturbing your tank.
- You can set aside some space in the sump for animals. (You might hear people refer to this space as a
refugium, though people don't always mean this as a part of a sump.) This can be a grow-out space, isolation, acclimation, etc. Bear in mind, though, that a sump is not usable as a hospital or quarantine tank, since it's part of the main system. Likewise, you can grow plants or algae in the sump. This can be done to keep nitrates down, or just as more grow-out space.
- You can actually have a sump supplying more than one tank, assuming you don't mind them sharing the same water parameters.
Sumps do have some potential risks you have to think about.
- If the overflow is just an open drain, you'll want a screen of some kind to keep fish out of the sump...
- ... but you also need to keep the overflow clear of obstruction at all times. Otherwise, you'll be pumping water into the tank faster than it can drain out the overflow, and that can quickly turn into a flood if you don't catch it.
- You need to keep a minimum volume of water in the sump for the life support to operate, particularly the pump and the heaters or anything else that can't safely operate when dry. So you have to be careful when you're doing a gravel vacuum -- it's easy to lose track of how much water you've taken out, since the tank's water line doesn't drop.
- The sump shouldn't be filled 100% though, in case the pump stops pumping for whatever reason. It has to be able to hold the extra water that will drain out of the tank when that happens -- which probably won't be a whole lot, but remember that water will need to go somewhere.
- And generally speaking, any point in your system where two things meant to hold water come together is potentially a point that can leak. Sumps add a whole lot of these points: the overflow from the tank, any plumbing on the drain or supply, etc.
The complexity and cost of a sump doesn't make a lot of sense with smaller tanks. I wouldn't bother with one unless I were setting up a tank so large I'd need multiple canister filters to provide adequate filtration. But in those cases, they really can be worth the effort. I've seen pre-built sumps for sale, but they're not cheap; I haven't actually done the math but I suspect most people who are at least a little handy would save money DIY'ing it -- there are designs all over the internet.
There's no reason you can't use a sump on a freshwater, but you'll most often see them in large saltwater setups -- salt water systems often have more life support equipment than freshwater, like protein skimmers and UV sterilizers, and it gets complicated trying to work all of that without a sump. And most home aquariums are small freshwater tanks that do fine without one.