The Basics of Aquarium Filtration

THE BASICS OF AQUARIUM FILTRATION

Dr Mike Statham

A lot of fish keepers are confused by the workings of the filters they use, which can lead to poor decisions when it comes to choosing a filter and maintaining it properly. This often leads to the death of fish through "poor water quality", which leads to frustration, a mistaken belief that fish keeping is "hard" and the departure of yet another would be fish keeper from a fascinating hobby. Sadly, all of this is easily avoided if some effort is made to understand why we use filters and how they work, this is the ONE secret of successfully keeping fish and really isn't that complex or difficult at all.

Why do we need filters?

Good question. In ponds, lakes, streams and rivers fish live very well without any need for filters. But, the key point here is the volume of water the fish are living in. Most of us can't afford to have pond sized aquariums in our house to keep a few small fish, we might be willing to do this in the garden, but not indoors. So our fish are always going to be more crowded than nature intended. This presents a problem.

Lets take an analogy, say cows. If you have 20 cows living in a field they are going to produce a certain amount of cowpats, but the field is a big space and the cowpats they produce are going to be pretty spread out. If they leave a pile of dung in one part of the field they can then move on to another part of the field and so on, until by the time they get back to the part with the dung, bacteria have done their work and it's all been broken down into harmless soil. Now put those 20 cows in a cowshed and things very quickly get messy, without the farmer to shovel the muck out and hose it down every couple of days the cows would soon by very ill. Very few animals really like living in their own excrement, it's full of harmful bacteria and could make us ill - would you want to live in your own poo?! Fish are no different.

Okay so that explains the "solids", but one of the big problems with fish in captivity is that the biggest quantity of waste they produce is invisible. There is one chemical in particular that all fish produce and have to dispose of, ammonia. Fish excrete ammonia via their gills, we (mammals) don't produce ammonia, but we produce the very similar urea instead, we store it up and get rid of it as urine. Fish are slightly different and usually expel ammonia from their gills along with the carbon dioxide produced by breathing. Concentrated ammonia is nasty stuff (think of bleach), it's not something fish want to swim around in and if the concentration gets to great it'll quickly kill them. In nature this isn't a problem as the fish are in such a big volume of water the ammonia is highly diluted and the bacteria break it down to less toxic chemicals (nitrites and then nitrates) before it ever gets concentrated enough to be a problem. But, in a fish tank it is a big problem, especially as it can build up fast, and the more overcrowded and stressed the fish the faster it builds up!

So, having defined that the problem with keeping fish in aquariums is that they are overcrowded and therefore the natural systems are inadequate to deal with the waste they produce, we now see that some form of filtration is going to be necessary to tackle this waste. A filter in a fishtank is simply a sewage works. We (humans) also live in much more crowded conditions than nature intended for us, most of us live crammed together in towns and cities, so we also need a helping hand to get rid of our waste - for which purpose we invented sewage treatment. In fact human sewage treatment and the filters we use in fishtanks use many of the same principles.

How do filters work?

There are basically only three different forms of filtration used in aquariums (and ponds for that matter):

Mechanical Filtration: This is the simplest form of filtration, it consists of some form of trap, such as a sponge or layer of cotton wool, in which solid particles get stuck allowing them to be removed. Mechanical filters need to be cleaned and/or replaced very frequently (daily/weekly/monthly), they are good at removing solids, but do nothing for dissolved wastes and quickly get clogged up with the dirt they collect. Mechanical filters are nearly always a fine sponge or "filter wool". They are usually coloured white (until dirty!). Mechanical filters are often used in conjungtion with Chemical and/or Biological filtration, as a first layer, to protect the Chemical and Biological media from getting clogged with dirt.

Chemical Filtration: This form of filtration relies on a chemical reaction to bind the waste to it. The most commonly encountered one in aquariums are Carbon based (charcoal, activated carbon etc...). So they are often coloured black, although different chemicals may be used so they may be a white powder, green sponge etc... Many people keep some activated carbon for use as a Chemical filter media, it is available as chunks of raw material and as extruded pellets. Chemical filters can only usually be used once and then must be disposed of, if they aren't then all the waste attached to them will be leached back into the water - making the entire process pointless. Some (resins) can be recharged by washing them in saline solution, but they still usually have a limited lifespan. Chemical filters are very effective, work instantly and will tackle dissolved wastes, but need constant replacement (weekly/monthly) so can be expensive/time consuming to use. These days it is rare for them to be used as the main filter.

Biological Filtration: This form of filtration relies on the natural processes by which various bacteria break down waste into less toxic forms. It is generally the best form of filtration, as it is cheap, highly efficient and self perpetuating. It can break down solid and dissolved wastes, but is usually used along with mechanical filtration to trap solid particles to speed up the process.

Although the best filtration overall, biological filtration has two big weaknesses. The first is that populations of filter bacteria take time to get established and build up to useful levels - the so called "maturing", or "cycling" of a new tank is merely this time and commonly lasts 2-3 weeks, or more. Some readily available filter "boosting" products can help speed this process.

The second is that the bacteria are quite sensitive and easily killed off. They will start to die off if they don't get enough oxygen (being "aerobic" bacteria they breathe in oxygen and produce carbon dioxide), this can result from a prolonged powercut, or switiching the power to the filter off (people often switch "noisy" filters off overnight without knowing what harm they are doing). Very commonly people wash out the biological filter media in tapwater, we add chlorine to tapwater precisely to kill off bacteria (cholera!), so you must NEVER wash a biological filter media in tapwater, always rinse it out in dirty tankwater instead. Finally, and often unavoidably, many treatments added to the tank to treat sick fish will harm the filter bacteria (especially anti-bacterials!). In all these cases it is useful to have some chemical filter media to hand for such emergencies until the biological media can be "matured" again.

Juwel, who colour code their filter pads, colour their biological pads blue, but they can be any colour. Two popular forms of media are a semi-rigid sponge, and some form of microporous ceramic (cat litter, ceramic rings), in big filters brushes and/or plastic balls ("bio balls") may be used instead. The only important criteria is that whatever media the bacteria live on should have a big surface area which comes into contact with flowing water. The best (and most expensive) biological filter media are those that have the maximum possible surface area allowing more bacterial filtration to occur in a smaller volume of filter media, these are typically sintered ceramics such as SERA Siporax.

Water changes

A water change is simply taking a percentage of the water in a tank out and replacing it with the same quantity of "clean" water. It can be viewed as a very basic form of filtration, and actually is a very good subsidy to having a good working filter if you want to keep your fish and plants healthy. Before filters were developed for use in fishtanks people used to rely on water changes to keep their pet fish alive - incredibly some people still keep Goldfish this way - this is NOT recommended!

In the modern (filtered) aquarium, water changes still have a role, they give some big advantages (dilution of waste, introduction of new minerals, refreshment of the fish), and few disadvantages (effort, controllable mess), so a small weekly water change is very highly recommended even for tanks with excellent filtration.

A good aim for water changes is to change 10-25% of the water every week, it won't matter if you miss one, or two changes, but aim for once a week. You can do it at the same time as cleaning the filter out and it needn't take more than a few minutes. It's especially important in small tanks (less than 100 litres) and for those 25% is a good quantity to change, in larger tanks smaller or less frequent changes are more practical and adequate. There is a simple rule that the larger the tank the less maintainence and cleaning is required.

The simplest method to change the water in a tank is to syphon out a bucket of dirty water, then pour another bucket of clean water back in. Syphoning is simple and doesn't require more than a length of tubing and a bucket, but there is a technique to it which make may take some mastering. For those who prefer them there are all kinds of kits and gravel cleaners that make the task easier. When replacing the water please ensure the clean water going in is dechlorinated and warmed to room temperature (in the winter tapwater can be very cold) before use, this is less important in larger tanks than it is in smaller ones as the new water dilutes faster in a bigger volume and will have less of an impact. Dechlorinators can be bought or the water can be stood in a bucket for 24 hours to degas (and warm up) naturally. If you have chloramine in your tapwater a chemical dechlorinator that can deal with it is essential, chloramine degasses from water much more slowly than chlorine (which is why it is used in some places). If you are using water which has been chemically treated or dimineralised, then obviously any water used in changes will need the same pretreatment.

What is the best type of filter?

This is impossible to answer, there are now literally hundreds of different aquarium filters on the market, and many serious aquarists assemble their own, most have advantages and disadvantages according to your needs (and wallet, but be warned cheap initially does not necessarily mean cheap in the long run). Some of the more popular are:

Internal Power Filter: These days most people come across these first. A little box that sits in the corner of the tank and sucks water through sponges by means of an impellor ("powerhead"). They are relatively cheap to buy, but require regular media cleans/changes to keep them working efficiently. Okay for planted tanks (without any additional aerating fittings: spray bar, venturi!). Can be annoying for use with big fish, crowded tanks, fry, as they need regular cleaning out and some (poor designs) can also suck up and mangle small fish. A good design is perhaps the best filter for most small to medium sized tanks. One design of these I strongly recommend is the brilliant Eheim Aquaball range.

External Power Filter: Very popular with bigger tanks and/or bigger fish. Initially expensive, and can be a little fiddly to setup, but in the long term relatively easy and low cost to maintain. Low turnover ones are ideal for planted tanks, high turnover ones with spraybars are not, as they aerate the water too much. Recommend Eheim ECCO range for excellent cheap option. Sponges they come with are best replaced with good sintered filter media (SERA Suporax) in the long term, especially if bioload is going to be heavy. Very rarely malfunction, but it is essential to make sure the lid is clipped on properly before use. You will also require space under the tank (or nearby) to house the filter and gain access for cleaning/maintainence. Not safe for use with tiny fish/fry - which can sometimes be rescued from the filter bucket.

Trickle Filtration: A variant on the theme of an external filter, the water is pumped to the top of a filter on top of the tank, through which it trickles, to finally fall back into the tank from the bottom of the filter. The very rarely seen (in this country) "hang on filter" is a variant on the same theme which hangs on to the outside of the tank.

Sump Filtration: Another variant on the theme of an external filter really. Often home made, but requiring some skill in plumbing and knowledge of filtration, so not recommended for beginners! Water is taken from the main tank to a secondary tank, where it is filtered using media (or sometimes bacteria filled mud and/or algae/plants - a so called "living refugium"), it is then pumped back to the main tank. Devising and building such a filter requires research and patience.

Internal Air Driven Sponge Filter: Have to admit these are a personal favourite (also of many other longterm aquarists). Bit of an unsung hero as they don't make much money for the manufacturers. You MUST have a separate air pump and airline. Very simple, consists of a "sponge on a stick". Air from the pump creates bubbles which rise up the tube (stick), sucking water through the sponge, which is both mechanical and biological media. Cleaning is just a regular squeeze in dirty tank water - sponges last years. Good for planted tanks, good for fry tanks, not good for big fish, and certainly not for crowded tanks. Recommend SERA range of these for value for money.

Box Filter: These haven't been popular for a long, long time now, but were the first real filters available (they have a prehistoric vintage), some people still use them - especially serious aquarists who often make their own for use in hospital and/or emergency tanks. The box filter consists of a plastic box (as the name would suggest), often divided into two parts, with slots or holes in the sides and a plastic pipe in the middle or a hole in top. Air from a separate air pump is sent down a tube to the bottom of the box where it bubbles out of an airstone, rising through the pipe/hole in the top of the filter through the filter media. As the bubbles rise they suck water with them, moving water through the filter. The usual filling for a box filter is a layer of filter wool (mechanical filter) and a layer of carbon (chemical filter). They have the advantages of being cheap, easy to assemble, biologically inert (water treatments won't stop them working) and instantly active (no maturation time). The disadvantages are the usual ones for chemical and mechanical filters (lots of maintainence, replacement of media). Thus they are best suited to emergencies, but then extremely useful. They are also good for transporting fish (with a battery air pump) and great for fish collecting expeditions - be it to Borneo, or Birmingham!

Undergravel Filter: Also out of favour these days, 20 years ago everyone used them. Consists of a big plastic plate that sits on the bottom of the tank under the gravel (hence name). Clear plastic "uplift" tubes in the corners have airlines from a separate pump generating bubbles, these bubbles suck the water down through the gravel and the plate. Some people replace the air pump with a "powerhead" (impellor) on top of the uplift tubes, this makes it all work faster. MUST be used with pea gravel to work, the gravel bed becoming the biological filter media, cleaning out performed by cleaning the gravel periodically. Rooted plants hate the water movement through their roots (okay with plants in pots/on bogwood/floating). Safe for use with fish fry, okay for crowded tanks. Cheap and easy to maintain, but limits choice of substrate/plants. Not as popular as they ought to be, perhaps because shops don't use them any more, having been replaced by:

Centralised Filtration System (CFS): A number of tanks are connected via pipework to a single big "sump", from which the water is pumped back to the connected tanks, i.e. an "external filter" for several tanks. These are popular with shops these days and with people with fish houses, there is quite a lot involved in setting them up properly - not a basic task. On the plus side they are VERY easy to maintain and make water changes easy, they are also relatively cheap to run. On the down side as they are all connected they can increase the spread of infectious parasites/diseases, something which many shops haven't handled very well, relying too much on the use of inline UV supressants and seeing them as an alternative to properly trained staff (in my personal opinion). They can also malfunction if not assembled/designed well and such "minor" problems can take a lot of work to fix once up and running. A very good understanding of the basics of filtration (and plumbing!) is essential before attempting to assemble one of these. In my opinion best reserved for highly experienced fish keepers/breeders with lots of tanks where they can be a very practical labour and cost saver and the risk of disease is properly handled by strict quarantine procedures.