Good filtration is essential for a healthy reef aquarium. In this beginner's guide, Jeremy explains the three main types of filtration – mechanical, biological and chemical – and how they work together to keep your water clean and your fish and corals healthy.
If you have kept fish before then you'll already know a bit about filtration. Good filtration is at the heart of a successful reef aquarium. Fish, corals and other reef inhabitants constantly add waste to the water, while uneaten food and other organic material can quickly build up, just like in a freshwater fish tank.
But unlike a freshwater aquarium, which can sometimes run quite happily on a single internal or external canister filter, reef aquariums use several different filtration methods and products to keep the water clean.
These filtration methods can be divided into three main types: mechanical, biological and chemical filtration. Each performs a different job, and together they create a cleaner, healthier environment for fish and corals.
More from our Beginner's Reef Guide:
Photo by Shutterstock
Mechanical filtration
Mechanical filtration is probably the type of filtration most fishkeepers are already familiar with. Its job is simple: physically remove particles from the water before they can break down and pollute the aquarium.
Common mechanical filter media include:
- Filter socks
- Filter wool (also known as filter floss)
- Roller filters
Photo by D&D The Aquarium Solution
The particles trapped by mechanical filtration are typically uneaten fish food, fish faeces, pieces of algae and other debris. You don't want this material sitting in the aquarium, where it can decompose and contribute to poor water quality – or just look unsightly.
The important part is that mechanical filtration doesn't make the waste disappear – you need to remove the trapped waste from the system. This means throwing away used filter wool or roller-filter material, or washing filter socks regularly.
For the best results, mechanical filtration should be the first stage of your filtration system, trapping larger particles before water reaches your biological and chemical media.
Biological filtration
Biological filtration is arguably the most important type of filtration in any aquarium. It relies on beneficial bacteria to process the toxic waste produced by fish and other organisms.
Fish produce ammonia, primarily through their gills, and ammonia is highly toxic to them. In the ocean, the enormous volume of water rapidly dilutes this waste. In the confined environment of an aquarium, we need beneficial bacteria to deal with it.
These bacteria establish themselves on surfaces – including coral sand, reef rocks, and rubble – throughout the aquarium and convert toxic ammonia into nitrite and then nitrate. This process is known as the nitrogen cycle.
Reef rubble – Photo by Shutterstock
Biological filter media
A wide range of biological media is available, and they all do the same thing to some extent. Their job is to provide a large surface area for beneficial bacteria to colonise, so the media is typically very porous to increase space for bacteria to live.
The bacteria don't only live on dedicated filter media, though. The rocks and coral sand also become coated with beneficial bacteria, making the entire aquarium part of the biological filtration system.
Live rock or dry rock?
Traditionally, many reef aquariums were started with live rock taken from the ocean. Because live rock is already covered with bacteria and other organisms, it would provide an immediate biological foundation for a new aquarium (if properly cured). Live sand was also commonly used to introduce beneficial bacteria.
Today, many reefkeepers choose to start with dry rock and dry sand, then add dedicated biological media and bottled bacteria to kick start the cycling process.
There are many bottled bacterial products available, and a reputable marine aquarium store can advise you on which product to use and how to cycle your particular aquarium.
Photo by George Farmer
Live rock and live sand are still available, but they are much more expensive. If you'd like to use live rock make sure you head to a trusted marine aquarium shop and ask about their curing process to ensure you're getting high quality live rock. Mature live rock will often have some patches of coralline algae (the pretty pink stuff pictured above), so if you spot this it is usually a good sign.
Keep an eye out for pests too, a live rock vat full of Aiptasia anemones for example is best avoided unless you're rearing an army of peppermint shrimp.
Related article: Marine Hitchhikers: Natural Pest Control for Reef Tanks.
Chemical filtration
Chemical filtration works differently from mechanical and biological filtration. Rather than physically trapping particles or relying on bacteria to process waste, chemical media use adsorption to remove or bind substances from the aquarium water.
Chemical media can help remove things such as:
- Unwanted organic compounds
- Dyes and discolouration
- Odours
- Phosphate
- Other unwanted chemicals
Phosphate remover
For reef aquariums, phosphate-removing media, such as RowaPhos, can be particularly useful.
Phosphate enters an aquarium through sources including fish food. If phosphate accumulates, it can encourage nuisance algae growth and interfere with coral growth.
Natural coral reefs generally have extremely low levels of phosphate, so maintaining appropriately low phosphate levels is an important part of creating a healthy reef aquarium.
However, you don't need to chase zero phosphate. Corals and other organisms need phosphate, and completely stripping it from the water can cause problems. Reefkeepers often aim to keep phosphate levels below 0.1 mg/l (0.1 ppm), but the ideal level depends on the aquarium, its inhabitants and overall nutrient balance.
Other ways to control phosphate
Phosphate can also be removed biologically.
Bio-pellets provide an organic carbon source that encourages the growth of bacteria which consume nitrate and phosphate. Bio-pellets are normally used in a fluidised reactor, where water is passed through the pellets to encourage bacterial activity.
However, bio-pellets can be extremely effective and, if poorly managed, can reduce nutrients too quickly. As with all nutrient-control methods, the aim is to control nutrients rather than strip them completely from the aquarium.
Growing algae can also help control phosphate. Just as plants and algae use phosphate as they grow in freshwater aquariums, macroalgae can be grown deliberately in a reef system for this purpose.
Photo by Shutterstock
An algae refugium, or dedicated algae reactor, can be used to grow macroalgae (seaweed) in the sump. As the algae grows, it takes up nitrate and phosphate from the water. A refugium can also provide a home for small organisms such as copepods, which can become a useful natural food source for reef inhabitants, which is especially good for fussy feeders like mandarins.
Some people will even add macroalgae into their main display for an attractive "planted" marine aquarium look.
Activated carbon
Activated carbon is another extremely useful chemical filter media. It helps polish aquarium water, removing compounds that can cause yellowing, making the water appear crystal clear.
It can also remove a variety of unwanted substances and contaminants, making it particularly useful in an emergency. Many reefkeepers choose to run activated carbon continuously, while others keep it available for situations where it may be needed.
Related article: Beginner's Guide To Reef Tanks: Why Water Flow Matters.
Combining the three types of filtration
The best reef filtration systems use a combination of mechanical, biological and chemical filtration, with each type performing its own role.
A typical setup should have:
- Mechanical filtration first – to trap and remove solid particles.
- Biological media in an area of good flow – allowing plenty of aquarium water to pass over the beneficial bacteria.
- Chemical media with strong water flow – often inside a media reactor, where water can be forced efficiently through the adsorptive material.
Building a clean, healthy reef aquarium
Effective reef filtration isn't about relying on one piece of equipment. Mechanical, biological and chemical filtration all have different jobs, and using them together gives you much greater control over the water quality in your reef aquarium.
Start by trapping solid waste with mechanical filtration, establish a strong biological filter to process toxic ammonia, and use chemical media where appropriate to control substances such as phosphate and dissolved organic compounds.
There is also another important piece of equipment that can take reef filtration even further: the protein skimmer.
Read our guide to protein skimmers to find out how they work, what makes a good skimmer and how to get the best performance from one.