Rocks or filters?
While that might not sound a fair comparison, rocks and filters in the tank can do pretty much the same thing – one perhaps better than the other!
Fish produce waste, just as we do, in the form of urine and faeces. They also excrete waste into the water directly out of their gills. in the closed environment of an aquarium, that waste has nowhere to go, so it builds up to toxic levels, poisoning the inhabitants.
The main type of aquarium filtration to deal with waste is biological filtration. This method utilises bacteria that use fish waste as a food source, turning it into something less harmful.
in the first instance, fish produce ammonia. uneaten food and other decomposing matter also contribute to the build-up of ammonia. And ammonia is deadly, even at the lowest levels.
Biological filters act as a home for bacteria that convert this lethal ammonia into lessharmful nitrite, and eventually into nitrate.
These bacteria are really slow to develop. it can take weeks or months to develop a colony of them sufficiently large to deal with the waste in even a moderate-sized tank. so, the bacteria need to be in place and coping before any livestock is added to the tank. This is done through either the addition of live rock, or through the maturation of biological filters, such as external canister or sump filters.
one annoying offshoot of biological filtration is that it can only convert waste so far – usually to nitrate – and this chemical still needs to be controlled through regular water changes.
Live rock is the ‘go to’ filter medium for the modern marine tank. Live rock is formed from old, dead coral skeletons, and naturally occurs abundantly around reefs. here, it behaves as a biological filter for the sea, housing the necessary bacteria required to convert wastes.
By harvesting this rock, aquarists can bring ‘filters’ straight from the sea and into their tanks. While expensive – even cheap live rock will set you back double figures per kg – it’s considered by far the best way to maintain water quality in a marine tank.
As a bonus, live rock goes above and beyond the activity of standard biological filtration. Because the rock is so porous, the bacteria penetrate deep inside it, meaning additional types of bacteria can occur. Among these are bacteria who will even use nitrate as a food source, converting it into harmless nitrogen gas.
As an extra bonus, live rock also comes with a bounty of other life forms covering it! Alongside colourful algae (especially the gorgeous, purple encrusting types), live rock can come with tiny polyps, corals, shrimps, crabs, sponges and more.
unfortunately, that also means you sometimes have a nuisance creature turn up as well, such as a predatory Mantis shrimp, or a coral-eating worm but, on the whole, most live rock is safe.
When buying live rock, make sure you only buy ‘cured’ live rock. After collection and transit, live rock turns foul for a short while and requires soaking and maturation to flush out any nasties within it – this is the curing process. While cheap, uncured live rock is available, if you add this to your tank it will almost certainly cause extreme water quality issues.
Always ask if it’s cured Before you buy!
Biological canister filtration
You’ll be familiar with biological filters if you’ve ever kept freshwater fish. They’re the canisters that sit inside or outside an aquarium, contain different types of media, and need regular cleaning and maintenance.
in marine tanks, canister filters have
largely been made redundant by super-efficient live rock, but some people do still like to use them – particularly those with fish-only systems.
The main drawback with a canister-type filter in a marine tank is that it doesn’t have the ability of live rock to convert nitrates into nitrogen (unless expensive supplementary filters are added). This means it’s more difficult to control nitrates, and will require very frequent water changes to dilute the nitrates back down.
Canister filters can also become dirty, so the media inside them becomes smothered and loses efficiency. In the event of a failure of the pump, the bacteria inside may starve or suffocate, leading to the filter ‘crashing’ and being unable to convert toxic wastes.
Biological filters also need to be cycled (see below) before fish can be added to the tank. This is time-consuming and frustrating in a way that live rock just isn’t.
What is cycling?
Cycling is the act of taking a filter without any bacteria and colonising it with enough bacteria to cope with the waste that will eventually be produced by fish.
The main way of cycling a filter involves adding liquid ammonia to simulate fish waste over a long period, providing the bacteria with a food source to grow on. Because the tank will be highly toxic with ammonia throughout this period, fish and corals cannot be added.
While cycling is a long-winded process, adding biological supplements can help. These liquids and tablets contain colonies of live bacteria that settle in the filter. Some claim to be able to mature a tank instantaneously, but it’s best to take such claims with a pinch of salt.
During the cycling period, test the water regularly to assess how much pollution is being converted, and therefore how the bacteria are developing. There are helpful calculators online where you can upload your test results. They’ll then tell you how much more ammonia to add until your tank is finally ‘mature’ and safe for fish.
Left: A typical canister filter.
Above: Biological supplements kick start filtration.
Above: Live rock brings a reef tank to full glory.
Below: Fresh live rock is rich with life and colour.