So how exactly is salt water tanks filtered?

[biofish]

So, one of reasons (other than y know…. PRICE) I have never ever had a marine tank is because of how complicated the filter system is. When I mentioned this to my LFS conversationally, I got two different responses.

My super expensive LFS: Salt water aquariums a second tank full of I can’t for the life of me remember but I think it was some type of live rock or special plant or SOMETHING. All I know is they opened up the bottom of one of their cupboards to show me their set up and I had a stroke trying to figure it out. Granted, the tank they showed me was a MASSIVE one.

My not very local LFS: said that salt water tanks, up to 100 gallons, just use the same filter as a freshwater tank.

And I was just wanting some clarity on which setup it is?

 

[JuiceBox52]

It’s called a sump, and can be done with or without depending on certain factors

 

[Lynnzer]

I just built an internal sump behind a black perspex divider. Easy enough, and in fact you can see this arrangement on many marine tanks in the shops.
I also have a 2nd tank that uses a canister filter from a previous freshwater setup.
The main thing with filtration is that it's there to remove bits and pieces of leftover food, fish waste etc. The actual chemical control of the water is done by having lots of live rock and a good amount of beneficial bacteria. This clip is just one from the same person that gives a good oversight on all of this.

 

[threecharacters]

There isn't a fundamental difference between filtration in saltwater and freshwater. However, saltwater tanks typically have much more complicated filtration systems because you want to avoid large and frequent water changes at any cost. Mixing saltwater is a pain, and the salt can be quite expensive especially if you are doing sizeable water changes on large tanks. Because of this many saltwater tanks have elaborate multi-stage mechanical filters. Examples are filter floss, filter socks, rollermats, and protein skimmers. Like any tank, they also have biological filtration media, which is often porous rock/rubble similar to what is kept in the tank. Things like carbon are used more often as well, and granular ferric oxide (GFO) is used to remove phosphates from the water. They'll grow algae like chaeto algae to reduce the nutrients in the water too. A more complicated version of this is an algae scrubber, which is just a device designed to grow algae efficiently to remove nitrate and phosphate from the water. This is just the basic stuff, there are more esoteric filtration methods like sulfur denitrifiers which are sometimes used by a subset of reefers.

A lot of this stuff would work perfectly fine in freshwater, even protein skimmers though they would have to be tuned/designed slightly different. Most freshwater aquarists find it simplier/cheaper to just do large, frequent water changes. Some freshwater aquarists with very very large tanks (like thousands of gallons) do use more complicated filtration methods like these.

Because saltwater tanks typically have more types of filtration they need a space to store all the equipment, which is where a sump comes in. Sumps allow you to hide a bunch of equipment that would clutter up the display tank. It also increases the total volume of the system and provides a protected breeding ground for live foods like copepods and nutrient sinks like chaeto algae. You could have a freshwater sump if you wanted, and I think i might even be a good idea for some of the larger tanks.

Saltwater doesn't have to be more complicated. I have a 12 gallon reef tank and the only filtration is some large pore sponge for biological filtration. The tank is small enough that regular large water changes are pretty easy and not too expensive so I can run it basically like my freshwater tanks.