This is part two in a series about how you can take the complexity out of keeping a saltwater reef tank. Make sure to catch part one, covering aquarium chemistry, to get up to speed.

Now that you're familiar with all of the different numbers for a reef tank and how they work, it's time to talk about filters. As we talked about before, filtration in the marine and reef environments is considerably different from what you'll find in freshwater. But just because it's different doesn't actually mean it has to be more difficult. There’s a lot to talk about here, but the concepts are really very easy. So without further ado, let’s dive in.

What you need to know first off is that water in the tank gets dirty. There’s leftover food, waste from the fish, waste from the corals…and it’s all just there floating in the water. We need a way to get that stuff out. We do that by filtration.

There are three types of filtration that will happen in your system, whether you're talking saltwater or freshwater:
Mechanical filtration happens via skimmers, filter pads and water changes. Biological filtration will be handled by your live rock and sand, and it's the biggest part of your filter. Chemical filtration happens when you push your water through something like carbon or granular ferric oxide. Each of them is important, but they don't have to be difficult.

The bulk of your filtration in a reef system will happen as water passes through the rock and sand that you have used to set up your aquascape (don't worry, aquascape is just a fancy word that we use for "placing your rock and sand"). In a freshwater system, the good bacteria will live in your gravel, but it primarily lives inside of a filter that you've added to the setup. Because reef tanks require significantly more stringent filtration, it's important that you're using the right kind of rock and sand, as well as making sure that you have enough water movement.

Get Moving

First thing's first: we have to get water moving through your rock to start establishing your biological filtration via the nitrogen cycle. As a general rule, how much water movement you need in your tank will depend on the types of corals that you want to have. Soft corals, for instance, have somewhat lower requirements. But if you want a tank that is primarily dominated by SPS (small polyp, stony corals, like acropora pictured below) you will need to have some pretty serious water moving around in the tank.

So how much flow do you need? Here's a pretty simple guide to remember: 
  • Soft Coral: 10-15x display tank volume per hour
  • LPS Coral: 15-20x display tank volume per hour
  • SPS Coral: 30x+ display tank volume per hour

Now you'll notice that I said "guide" as opposed to "rule". That's because you'll find people who are using only 15x turnover but they have absolutely stunning SPS corals. There are very few hard and fast rules in reef keeping and water flow definitely has a lot of flexibility.

But how do you get that much flow through your tank? It's really quite easy, because almost every pump, filter or powerhead that you'll put into your tank will have a flow rating on it. All we have to do is figure out the amount of flow we need, and then buy the equipment to make that flow happen in the right manner. Here's an example:
I have a 40 gallon breeder display tank, with a 20 gallon long that I use as a sump (we'll talk about sumps in a moment). I primarily have LPS corals in my tank (like the branching hammer coral you see below), but I do like to keep some SPS corals too. Because I cross the line between the two types, I want to aim for a flow rate that is somewhere between the high end for LPS and the low end for SPS. Looking at the list above, I settled on a number of around 25x my display tank volume. So 40 gallons, times 25 = 1,000 gallons per hour.
In order to reach that number, I had to find the right combination of pumps and power heads. I know that my return pump, which pushes water from my sump up to my display tank, moves around 350 gallons per hour. I still needed another 650 gallons per hour, so I added a Hydor Koralia Nano powerhead on the left and right sides of the tank that was rated for 425 gallons per hour. Though that put me above my 1,000 gallons per hour number, careful placement of the power heads allowed me to have more flow without causing a sandstorm or blowing over my more sensitive corals.
HOBs and Sumps and Canisters, Oh My!
Everything that we’ve talked about so far can be referred to as “internal filtration”. It’s the stuff that’s going on inside of your tank. But almost without fail, the most successful reef hobbyists also use some type of external filtration. In smaller systems, using a hang-on-back filter is somewhat common. The Aquaclear series, from Hagen, is especially popular because its basket area leaves a lot of room to stack in the filter media that you would want to use.
Some reefkeepers choose to use canister filters, which are external filters that sit down on the floor. These tend to not be as popular of a choice, however, as it’s easy to forget about them and they’re often a bit of a pain to maintain.
Almost universally, the external filtration of choice in a reef environment is an external sump. A sump is a container, often a smaller aquarium or a custom device that looks like an aquarium, with dividers (called baffles) inside of it that can help to separate the different areas.  For example, take a look at this Berlin sump:

The area on the left is where the water will come from your tank’s overflow. The water first enters this area via a filter sock, which is a very fine piece of cloth that provides mechanical filtration. The area around the filter sock would likely contain some live sand and rock  (for biological filtration). The next area of the sump would likely contain a protein skimmer (for mechanical filtration). The water would then pour over and under the baffles (to remove air bubbles) before being returned to the tank via a return pump.

Sumps provide a number of benefits, but not the least of which being a larger overall volume of water in the system. If you think back to the previous post, you’ll remember that stability is key in a reef system. The more water you have, the more tolerant the system will be of changes. Think of it as a real-life “drop in the bucket” scenario.

Wrapping Up

Filtration in the reef tank can seem really complex, but it’s actually quite simple. It’s easy to get overwhelmed by the options and decisions, but keeping the basics in mind is what will help you to make an informed choice. In a freshwater tank, we let the filter cycle and then we decorate to our heart’s content. Only live plants really serve the purpose of both decoration and filtration. But in the reef, almost every part of the tank plays a part in the filtration dance.

You can make filtration in the reef tank as complex as you want, but often times adding in new pieces before you’re aware of how they work (and the effects that they’ll have) can cause the new hobbyist heartache. We’ll dive in deep with skimmers, media reactors and all kinds of other filtration devices in another post, but for now you have the basics that you need to know about filtering your reef tank.