Frequently asked questions on undergravel filtration


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Matt Clarke answers some of the most frequently asked questions on undergravel filtration.

How do undergravel filters work?

An undergravel filter is a slatted tray which forms a false floor that allows a layer of gravel to be suspended above a water filled space. One or more vertical pipes, called airlifts (or uplifts), are attached to the plate through which water is drawn using a powerhead or an airstone (powered by an airpump).

The suction created pulls water and solid wastes into the gravel and provides a source of oxygen-rich water for beneficial nitrifying bacteria, which remove the resulting pollutants from the water. This means that the filter functions both mechanically, by trapping debris, and biologically, by acting as a colonisation site for the bacteria that break down the fishes' wastes.

Are they any good?

Undergravels tend to be more labour intensive to maintain than internal filters, and can cause problems in planted aquaria.

However, they are cheap to buy and run, and do have a much larger surface area than most internal power filters. This means that they are theoretically capable of supporting a greater pollution load, once the bacteria have colonised the filter. When not looked after properly they can cause a number of problems.

How much gravel should I place on top? And what sort should I use?

Research suggests that the minimum depth should be 76mm/3", and the optimum gravel size between 2-5mm. If the gravel size is too small, or if you add sand to it, it will drop down and clog the slits, restricting the flow of water through the filter bed. As a general rule of thumb allow about 10lb of gravel per sq ft or roughly 45 kg per m2.

What flow rate do I need?

A flow rate of 60gph/272 lph per sq ft is best when the filter is at the optimum depth of 76mm/3". The flow rate should be higher if the bed is deeper.

Undergravels don't need cleaning - do they?

Undergravel filters do need cleaning to keep them running properly. If the dirt they suck in is not removed it will eventually slow the flow of water through the bed. This reduces the amount of oxygen getting to the beneficial bacteria, leading to poor performance and a deterioration in water quality. If the bed becomes blocked, toxic chemicals, like hydrogen sulphide could build up. Nitrate (and phosphate) levels can also escalate to extremely high levels if cleaning is insufficient.

How often should I clean the gravel?

The frequency of gravel cleaning really depends on how dirty it gets, and what the water quality is like. Tanks containing large messy fish need more maintenance than those containing a small number of tiny fishes. Although essential, thorough cleaning does probably have some impact on performance. According to some research, many of the bacteria present on an undergravel filter bed are attached to trap detritus. Thorough cleaning removes lots of detritus in one fell swoop, and can reduce the bacterial population by 40-66%, thus reducing nitrification.

However, regular cleaning prevents the build up of detritus, so the bacteria are more likely to attach to gravel. It's thought that a quick clean each week has less impact than allowing the detritus to build up over a month and then removing it with a gravel cleaner.

My filter seems to have clogged. How can I unclog it?

Infrequent gravel cleaning may lead to the slits under the plate clogging. Insert a siphon tube down the uplift and try sucking out the water from beneath the plate. If sucking alone doesn't work, try blowing gently down the hose to release the trapped debris and then start siphoning again.

If that fails, as a last resort, you may need to strip down the tank and scrub the plate with a toothbrush. Make sure you only wash the gravel in water taken from the tank. Never wash the gravel in tapwater or replace it with new, otherwise you'll lose the beneficial bacteria and the water will quickly become polluted.

Do they come in any size?

Undergravel filters come in a range of standard sizes made to fit snugly in the bases of the most popular tanks. If you have a larger tank you can use several plates, providing you add sufficient pumps to provide adequate flow through the filter. Some manufacturers make small plates that can be connected together to make custom filters for large or irregularly-sized tanks.

My plants don't grow very well. Is this something to do with my undergravel filter?

Unfortunately, few plants grow well in tanks containing undergravel filters. This is thought to be because they draw oxygen across the roots of the plant, and like other filters, oxidise nutrients and drive off carbon dioxide.

Choose plants that don't need to be rooted directly into the gravel, such as Anubias and Java fern. Or place specimen plants in clay pots partially filled with laterite, a nutrient-rich clay.

My cichlids expose the filter when they are spawning. Is that a problem?

When part of the plate is exposed, the water will flow through the whole and will bypass much of the filter bed. Underworld (01509 610310) make a special plastic mesh material called a gravel tidy, which can be cut to size and sandwiched between two layers of gravel. Digging cichlids can only dig down as far as the gravel tidy, rather than exposing the whole plate, which means that the filter remains functional.

This article was first published in the September 2002 issue of Practical Fishkeeping.