How To Choose The Best Amazon Aquarium Undergravel Filters?

Undergravel filters are a system that is not as popular now as in the past. With the improved design and lower price of modern canister filters, these are usually used instead of undergravels.

Undergravel filters are designed to utilise the nitrifying bacteria that exist naturally in the gravel of an aquarium. In order to do this, well-oxygenated water from the aquarium must be circulated through the gravel. This is achieved by having an undergravel plate with uplifts that is placed on the base of the aquarium and then covered by the gravel. This creates a water space underneath the gravel so that water can circulate. The uplift tubes, usually positioned in the corner or corners of the tank, are where water is pulled upwards by a powerhead (impellor driven pump) or airstone and air pump, forcing water to move down through the gravel.

Since the gravel provides a very large surface area for nitrifying bacteria undergravel filters can be efficient biological filters. Their efficiency depends on the type and amount of gravel and on the water flow through the gravel. Using an airpump and airstone is usually not effective as the water flow is not great enough to either turn over the water volume at the required rate, or to support sufficient numbers of nitrifying bacteria (remembering that these also need good water flow to provide them with oxygen). Powerheads are available in a range of sizes, a model with suitable turnover should be used for the size of tank.

The size of gravel used is important; too fine a gravel may impede water flow, or may fall through the undergravel plate, too coarse a gravel will not support enough bacteria. A grain size of 2-3mm is ideal. Generally aquarium gravels are no finer than this, but many (eg pebbles) are coarser. The layer of gravel should be at least 4-5 cm thick in order to provide sufficient filtration.

The biological filtration ability of undergravel filters will be impaired if they become clogged with organic wastes, and they do not remove solid waste very efficiently. An additional mechanical filter will be beneficial in tanks which have a high load of solid organic waste. Regular vacuuming of the gravel is important to remove any organic matter which is trapped in the gravel bed.

If water flow through the gravel is stopped for periods of more than a few hours (eg in the event of a power failure) the gravel may become anoxic due to the consumption of oxygen by heterotrophic bacteria and the nitrifying bacteria will be killed off. In the worst case, toxic hydrogen sulphide can start to form in the gravel. This is more likely if the gravel contains a lot of organic waste, so it is vitally important to keep the gravel bed as clean as possible. It is a good idea to do a water change and vacuum the gravel after a power outage to ensure the gravel is not anoxic.

Undergravel filters do not provide any easy method of incorporating extra mechanical filtration or chemical filtration. If these are required the use of an additional filter is recommended. Despite the common misconception, many plants will grow well in tanks equipped with undergravel filters. However, gravel fertilizers such as laterite cannot be used.

Undergravel filters may not be effective in tanks where digging fish are likely to uncover the plate, as the filter only works when the water is passing through the gravel. Using a thicker layer and/or slighly coarser gravel can help, or a grid or porous mat can be placed a few centimetres above the undergravel plate to prevent the fish from digging past this level.

Undergravel filters can provide adequate biological filtration for marine aquaria, however, they do have some drawbacks. The number one problem is disease. In a tank with undergravel filtration it is impossible to eliminate whitespot and velvet, even if ultraviolet sterilization is used, as cysts can persist in the gravel for many months.

Some sources recommend placing filter matting over the undergravel plate to prevent gravel falling through. This is usually not necessary. A small amount of gravel under the plate will not affect the filter, it is only important that water is able to move freely under the plate.

In “reverse flow” undergravel filters water is pumped down the uplifts by a powerhead and forced upwards through the gravel. This prevents organic matter from collecting in the gravel, but instead it collects under the plate where it is difficult to remove! If the water is pumped through the gravel after being pre-filtered (eg from the outlet of a canister filter containing mechanical filter media) then the undergravel can provide good additional biological filtration. Using reverse flow undergravel filtration without a mechanical pre-filter is not recommended.

Undergravel filters are best suited to medium sized and larger tanks. Small tanks do not generally have enough gravel area to make undergravel filtration worthwhile.

Getting the most out of this filter system:

• Choose an undergravel plate that covers most of the base of the aquarium and team it with a thick layer of fine gravel and a suitable 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).
• Vacuum your gravel regularly to remove organic wastes.
• Clean around the impellor of the powerhead every 2-3 months to ensure it runs smoothly, and replace the impellor if necessary.

Troubleshooting powerheads on undergravels:

Problem: Filter does not run, ie. impellor is not turning

Possible causes & remedies:
1. Check power supply, make sure filter is plugged in and turned on at the power
2. Impellor may be jammed: gentle shaking will sometimes start the filter, but the impellor or impellor well may need to be cleaned.
3. Faulty/worn impellor or broken shaft: check the condition of the impellor and shaft and replace if needed.
4. Burnt-out motor or broken wiring: these cannot be repaired, you will need to replace the filter.

Problem: Impellor turns but little or no water movement occurs

Possible causes & remedies:
1. Restriction of water intake strainer or prefilter: Perform gravel clean
2. Faulty/worn impellor: Check the impellor, ensure the fan is fixed to the impellor (some spin and click, but should only have about a quarter turn free play), replace impellor if needed.
3. Lift height too high for pump capacity: the higher up the water must be pumped, the greater the decrease in flow rate. Check the maximum head height, this should be at least twice the height to which you wish to pump.
4. Restriction of flow by outlet hose or attachments: if the pump is attached to a hose make sure it is of adequate diameter as narrow hose can resrict flow. Clean debris from the hose or other attachments with a flexible brush or pipe cleaner if clogged.

Problem: Filter produces excessive noise

Possible causes & remedies:
1. Faulty/worn impellor or shaft: check the condition of the impellor and shaft and replace if needed.
2. Restriction of water intake strainer or prefilter: Perform a gravel clean.

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