Crystal clear, healthy-looking water is the aim of virtually every aquarium owner. We are drawn to clear water as a species, we like to live next to it, swim in it, and of course, drink it.

But if not properly maintained aquarium water can quickly become cloudy, full of algae and discoloured. Dirty aquariums are one of the biggest reasons why people give up fishkeeping, so by keeping your tank clean, it will not only help the non-fishkeepers around you enjoy your tank, it will also help to keep you in the hobby, and enjoying fish.

Aquarium filtration

Filtration is the most fundamental way that we keep aquarium water clear. Filtration can be divided into three different types - mechanical, biological and chemical, and its mechanical and chemical filtration that work together to keep water clear. 

Mechanical filtration refers to the physical trapping of dirt and debris. Dirt particles suspended in the water are sucked into the filter where they are trapped in a sponge, filter wool or a combination of both. The finer the sponge, the smaller the particle it will trap, and filter wool, also known as filter floss, can further polish the water by trapping fine particles from cloudy water. 

Make sure you have a suitably sized filter for your aquarium. Find out the length and volume of the tank, and choose a filter model accordingly i.e a 90cm, 180litre aquarium will need a filter suitable for tanks of 90cm, and 180 litres and over. You can’t over-filter mechanically, so in order to achieve even cleaner, clearer water you could opt for the next filter model up, like one for 120cm, 240litre aquariums for example, or even double up and have one filter suitable for a 90cm tank, at each end. 

The more suction and filter media you have, the more water clearing capacity you will have, so large filters come in the form of external canister filters, which sit in the cabinet underneath the tank. Fine polishing pads are also available to further mechanically filter water, so if you’re shopping for a filter and want the clearest water, keep an eye out for a model with filter media options that include these.

Replace fine polishing pads, filter wool or wool on a regular basis, as they will clog up and reduce filter performance if left. Just throw them away and replace them with new ones. Sponges will also play an important biological role, harbouring beneficial bacteria which convert toxic fish waste, so only ever wash these in old tank water (not under the tap,) and if you need to replace them cut in half and replace half at a time, with an interval of at least six weeks in between, for beneficial bacteria to transfer.

Choose a filter model supplied with two sponges for better mechanical filtration, and ease of replacement without the loss of bacteria. 

Chemical filtration for aquariums

Chemical filtration works by absorbing or adsorbing things from the water. Activated Carbon is by far the best known chemical filter media and is widely used to get crystal clear aquarium water.

Crushed charcoal, aquarium carbon sucks up dyes and odours and locks it away inside, before being removed and replaced by the owner. It either comes as loose granules, in net bags or specially impregnated filter sponges, specific to certain filter makes and models. If it’s loose, you’ll need to contain it within a filter bag, and all carbon should be changed at least monthly as it will become saturated and lose its effectiveness.

Activated carbon is carbon that has been burned at a higher temperature, making it more adsorptive and more capable of clearing water. Carbon is especially popular for removing the yellow discolouration caused by bogwood and other aquatic woods. Use sponge, filter wool and carbon in combination to clear aquarium water.

Beneficial bacteria for aquariums

Some strains of bacteria can also be added to aquarium water to help to clear it. Bacteria can carry out an infinite number of different tasks in fish tank water including reducing waste, which would otherwise cloud water. Unique bacteria and enzyme cultures can be added to the tank and get to work reducing mulm and debris and clearing water. These can be added separately to start-up bacteria and for best results use on a regular basis.

Aquarium fish

Some fish are more conducive to a clearwater aquarium than others. Goldfish and cichlids are greedy, messy feeders which eat a lot of food and produce a lot of physical waste. They also dig in the substrate, dislodging fish poo and uneaten food which in turn produces cloudy water.

Overstocking can cause cloudy, dirty water, and a lightly stocked aquarium with a few small fish will nearly always have clearer water than one which is heavily stocked, with large fish. Large, messy fish should be accompanied by suitably sized filtration, and if you keep such species but have cloudy water, a larger, more capable filter should be chosen. 

Tropical fish food

The food you feed will also have a bearing on tank water clarity. Large, protein-rich foods like cichlid pellets, catfish tablets, frozen cockle and mussel may cause suspended particles, also called fines, degrading water quality and clarity, with the look of cloudy water. 

Be careful not to overfeed, remove any uneaten food with a net and/or gravel vacuum and match these diets to suitably large mechanical filters. A good flake food shouldn’t cloud the water so if you crave clear water and the flake refers to clearwater or low waste in its literature, that’s the one to try. 

Aquarium water changes

Maintenance and water changes are one of the best ways to achieve and maintain clear water. Clean filter sponges and vacuum the substrate regularly and you will remove physical waste from the system which would otherwise contribute to cloudy water. Weekly water changes also aid fish growth, keep nitrates down, and help to stabilise pH. 

Aquarium flocculants 

A good, short term way to get crystal clear aquarium water is to use a flocculant. Flocculants clear water by clumping tiny particles together, making bigger particles, which can then be removed effectively by filtration. Use a flocculant and clarity will temporarily get worse before it gets better, as all those tiny particles get clumped together before being removed. You’ll need a filter too, but they are quick, easy and widely used and are effective on green water and cloudy water.

Aquarium lighting

Have the tank light on for too long and you risk the water turning green. Tiny unicellular algae float around in the water, fuelled by light and nutrients, giving water the look of pea soup. If the cause of poor water clarity is a green tinge, it is almost certainly light and algae related, and the first thing to do is reduce the amount of light. Radically reduce the number of hours per day the aquarium is illuminated for, and if it receives direct sunlight at all during the day, if you can, move it to help fight the algae bloom. 

Ultraviolet clarifiers are widely used by pond owners to combat green water, and they can be used on aquariums too. If your tank suffers from green water invest in a UV and you won’t suffer it again. But if you want something quicker and less expensive in a single dose, use a flocculant, as recommended above.             
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