Undergravel filters are intended to use the nitrifying bacteria found naturally in the aquarium gravel. To do this, well-oxygenated aquarium water must be cycled through the gravel. This is accomplished by placing an undergravel plate with uplifts on the aquarium’s base and then covering it with gravel. This creates a water gap under the gravel, allowing water to flow. Water is lifted upwards by a powerhead (impellor-powered pump) or airstone in the uplift tubes, which are normally located at the tank’s corner or corners, pushing water to travel down through the gravel.
Undergravel filters may be particularly effective biological filters because the gravel offers a vast surface area for nitrifying bacteria. Their efficiency is determined by the kind and quantity of gravel used, as well as the movement of water through the gravel. Using an air pump and airstone is typically ineffective because the water flow is insufficient to either turn over the requisite volume of water at the needed rate or to feed a sufficient number of nitrifying bacteria (remembering that these also need good water flow to provide them with oxygen). Powerheads come in a variety of sizes; a model with an appropriate turnover should be utilized for the size of the tank.
The size of the gravel used is critical; too fine a gravel may obstruct water flow or fall through the undergravel plate, while too coarse a gravel may not sustain enough bacteria. The optimal grain size is 2-3mm. Aquarium gravels are generally no finer than this, although many (for example, pebbles) are coarser. To provide adequate filtration, the gravel layer should be at least 4-5 cm thick.
Undergravel filters’ biological filtering capabilities will be compromised if they get blocked with organic wastes, and they do not remove solid waste very well. In tanks with a high load of solid organic waste, an extra mechanical filter will be advantageous. Vacuuming the gravel on a regular basis is necessary to remove any organic debris that has been trapped in the gravel bed.
If water movement through the gravel is interrupted for more than a few hours (for example, in the case of a power outage), the gravel may become anoxic owing to heterotrophic bacteria consuming oxygen and killing off the nitrifying bacteria. In the worst-case scenario, poisonous hydrogen sulphide may build in the gravel. This is more likely if the gravel contains a lot of organic waste, so keeping the gravel bed as clean as possible is critical. After a power loss, it is a good idea to replace the water and vacuum the gravel to ensure that it is not anoxic.
Undergravel filters do not give a simple way to incorporate additional mechanical or chemical filtering. If they are necessary, an extra filter should be used. Undergravels, on the other hand, has tremendous biological filtering capacity when utilized appropriately. Plants will also thrive well in tanks fitted with undergravel filters, contrary to popular belief.
Undergravel filters may not be successful in tanks with digger fish since the filter only functions when the water passes through the gravel. A thicker layer and/or somewhat coarser gravel may assist, as might placing a grid or porous mat a few centimeters above the undergravel plate to prevent the fish from burrowing beyond this level.
According to some publications, filter matting should be placed over the undergravel plate to prevent gravel from falling through. This is seldom required. A tiny bit of gravel beneath the plate will have no effect on the filter; the only thing that matters is that water may circulate freely under the plate.
Water is pushed down the uplifts by a powerhead and driven upwards through the gravel in “reverse flow” undergravel filters. This prevents organic materials from accumulating in the gravel, instead accumulating behind the plate, where it is tough to remove! If pre-filtered water is poured through the gravel (for example, from the outflow of a canister filter containing mechanical filter material), the undergravel may offer good extra biological filtration. It is not suggested to use reverse flow undergravel filtration without a mechanical pre-filter.
Undergravel filters work well in medium and large tanks. Small tanks often do not have enough gravel area to warrant undergravel filtration.
Making the best use of this filter system:
Select an undergravel plate that covers the majority of the aquarium’s base, together with a thick coating of fine gravel and sufficient powerhead (Remember that good filtration requires turning your tank volume over three times an hour, more filtration may be needed if you have more or larger fish, or for marine fish).
To eliminate organic waste, vacuum your gravel on a regular basis.
Every 2-3 months, clean around the impellor of the powerhead to ensure it operates smoothly, and replace the impellor if required.