There are 2 different systems for undergravel filters:
- In the pump version water is pumped into the ground from below and flows through the ground from bottom to top.
- In the intake version water is sucked into the ground from above and flows through the ground from top to bottom.
In both versions, a bottom filter requires coarse gravel and coarse fertilizer. Sand and fine gravel are not suitable. The pump version is considered a better alternative because the water goes through the canister filter before it flows through the floor.
Soil filters create a constant circulation in the soil. This increases the oxygen content in the substrate.
Arguments for soil filters:
- Putrefactive substances are transported away from the plant roots.
- The flow rate of water can be regulated.
- In the pump version, no mulm accumulates in the ground.
- Larger amounts of mulm collect in the ground only in the intake version.
- Any mulm in the floor does not rot because the floor is always flushed a little.
- Nutrients are transported to the plant roots.
- Substrate fertilizer can be used if flow is kept low.
- The plants are growing very well.
- Practical experiences with soil filters are good.
Arguments against floor filters:
- Due to the increased oxygen content, nutrients oxidize and become unusable for the plants.
- In a dense substrate, e.g. B. sand with a grain size of 0 – 2 mm or gravel with a grain size of 1 to 2 mm, no putrefactive matter penetrates, which must be removed.
- Soil filters can only remove putrefactive matter at the beginning. The floor will clog over time. Then a few free channels are created through which all the water is pumped. In the congested and quiet zones, the dirt rots or ferments.
- Plants grow more poorly because the roots prefer an anaerobic, ie low-oxygen, environment.
- In order for nitrate to be broken down in the soil, anaerobic zones must form in the soil. Due to the strong bottom current, no anaerobic zones can develop.
- It is practically impossible to make the flow through the bottom filter so weak that anaerobic zones are created.
- Plant growth is poor.
- Algae are promoted.
- If fertilizer is applied in the soil as the first layer, the fertilizer is washed out through the soil filter and distributed in the aquarium water.
- Effort and result with soil filters are in no reasonable relation, because filters are no longer a practical problem.
Which floor structure is suitable
The substrate is just as important for an aquarium as the water. After the running-in phase, it has a very individual environment that depends on the structure and depth of the soil.
The bottom must be so tightly joined that no water flows in the bottom, but only a capillary exchange with the aquarium water takes place.
The upper three centimeters of such a substrate are populated with aerobic bacteria. Anaerobic bacteria settle underneath. The boundary is of course not quite as sharp.
This condition is only maintained if the substrate is not disturbed and z. B. is changed in its structure by underfloor heating or floor floodlights.
Therefore, only fine sand or gravel is suitable as a material. Other material such as B. coarser gravel, prevents the colonization of the desired bacterial cultures, because undissolved organic substances trickle through to the bottom pane. There they rot and poison the water. Among other things, blue-green algae can develop.
Underfloor heating is not inherently harmful, but the heating output should not exceed one watt per 100 liters of water. Otherwise the desired capillary effect is lost. For small aquariums up to 250 liters, such floor heating can be useful with a coarser substrate, because the ratio of the floor surface to the floor height is much smaller than in large tanks with large floor surfaces. The floor height should be at least 7 cm.
However, measurements with a thermometer in a room at 15°C and a water temperature in the aquarium of 25°C resulted in temperature differences between the soil and water of less than 1°C. It is then unlikely that plants will get cold feet and that floor heating will be necessary. This is supported by experience with aquariums without floor heating, which also showed magnificent plant growth.
Because large amounts of water get into the substrate from above or below through a soil filter and a real flow of water is created, such a soil climate can practically not arise with soil filters. The capillary effect, in which the aquarium water in the bottom becomes more and more oxygen-poor as it goes down, does not occur.
Anaerobic bacteria can hardly settle inside the soil. The soil climate created with soil filters does not correspond to the desired eco-biotope.
According to calculations of the oxygen or nitrate input into a sandy substrate with still water, only a few milligrams of nitrate are produced in a 100 liter aquarium per day due to denitrification in the substrate. The effect of nitrate respiration in the substrate, based on diffusion, is therefore minimal.
Because the filters normally sold in stores are typical quick filters, almost no biological filtration takes place because the flow rate in the filter is far too high for this. In most cases, a well-laid substrate of fine sand then takes over this task because its mass is greater than the mass in the filter bowl. The only thing that is effective on the filter is its mechanical screening effect and the pump that ensures circulation.
Gravel as a substrate is also a very effective additional filter. Gravel can also have denitrification properties, especially when suction filters are used. However, this gritty soil filter is not stable. There is a run-in phase, an optimal phase and an anaerobic phase. Maintenance measures lead to sudden changes in the mode of action.
Overall, sand or fine-grained gravel therefore seems more suitable as a substrate than coarser-grained material.
With coarse-grained substrate, soil filters are a possible alternative.