How to reduce the flow rate on an OASE BioMaster filter?

To reduce the flow rate of an OASE BioMaster filter, start by softening the water output (adjusting the orientation of the diffuser or spray bar), then use the adjustment provided at the hose connection, following the instructions for your model. Avoid restricting the intake or allowing the filter pads to become clogged: this will reduce the flow, but also the filtration efficiency.

BioMaster external filters are designed for demanding aquariums: the 600 model, for example, is suitable for tanks up to 600 litres with a maximum flow rate of 1,250 litres per hour. In a smaller, heavily planted aquarium or one stocked with fish that prefer calm water, this flow may seem too strong. As aquatic environment experts since 1998, we will explain how to adjust it effectively.

Why reduce the flow rate?

A high flow rate is not a fault in itself: it ensures good circulation, brings water to the filter, and prevents dead zones. However, certain situations justify moderating it:

• Fish that prefer calm water such as bettas, gouramis, or long-finned fish, which become tired in a strong current.
• A planted aquarium where fine stems lie flat and where an overly agitated surface accelerates the degassing of CO₂ supplied to the plants.
• A fine sand substrate that gets dug up by the jet.
• A filter chosen one size above the actual volume of the aquarium, which is common and often advisable for biological capacity.

Before adjusting the settings, make the right diagnosis. If the flow seems too strong immediately after maintenance and weak a few weeks later, the problem may not be the setting, but the gradual clogging of the filter pads.

First solution: soften the water output

This is the simplest method and the one that best preserves filtration, as the filter continues to operate at full capacity. Only the way the water returns to the aquarium changes.

• Aim the diffuser supplied with the kit towards the back or side glass: the jet breaks against the glass and disperses.
• Raise the output so that the jet just touches the surface. You maintain good gas exchange without creating a bottom current.
• Use a spray bar: the water exits through a series of small holes along its entire length, which divides the current speed.
• Place the output behind a decoration or a group of robust plants that break the current.

Second solution: adjustment at the connection

Most external filters, including BioMaster models, have a system at the hose connection adapter that allows you to cut off the water during maintenance and, depending on the model, regulate the flow. The exact procedure and markings depend on the version of your filter: refer to the instructions provided with the device.

A few principles to follow:

• Reduce gradually and wait a few minutes between adjustments.
• Prefer to adjust the discharge side rather than the intake: heavily restricting the intake makes the pump work under poor conditions.
• Check that the filter remains quiet: a new noise or air bubbles indicate a setting that is too tight or an air leak.
• Never completely close the passage while the filter is running.

The role of filter pads and maintenance

The BioMaster combines an EasyClean pre-filtration chamber, which catches large particles, and a biological filtration volume filled with Hel-X and sponges. When the pre-filtration pads become clogged, the flow rate naturally drops. This is normal, but it is not a good way to reduce the current: water passes through less effectively, waste accumulates, and efficiency decreases.

The right approach: regularly clean the pre-filter, which can be accessed without dismantling the entire filter, and rinse the pads in aquarium water rather than tap water to preserve the beneficial bacteria. Replace them when they become misshapen or no longer regain their shape.

What if the filter is oversized?

Choosing a filter slightly more powerful than the aquarium volume is common practice: you gain biological capacity and a safety margin. But if your aquarium is significantly smaller than the range intended for the filter, no setting will be fully satisfactory.

In this case, there are two options:

• Keep the filter and combine a spray bar, orientation, and moderate adjustment, which works well in most cases.
• Switch to a lower-capacity model in the same range, such as a BioMaster 350 for a medium-sized aquarium, and reserve the largest filter for a bigger tank.

Our article on the filtration flow rate for an aquarium helps you make the right calculation.

Installing a gentler output, step by step

Modifying the water output takes only a few minutes. Here is the method we recommend, whether you use the original diffuser or a spray bar compatible with the diameter of your hoses.

1. Unplug the filter and close the passage at the connection, as indicated in the instructions, to prevent the water from draining.
2. Remove the current output and identify where you want to disperse the water: most often along the back glass, just below the surface.
3. Fix the spray bar horizontally with its suction cups, holes facing the glass or slightly towards the surface.
4. Reopen the passage and reconnect, then let the filter purge the air for a few minutes. The purge button helps with a quick restart.
5. Observe the fish, plants, and surface movement for 24 hours, then adjust the orientation if necessary.

A slightly rippled surface that is not agitated is generally a good compromise: the water exchanges gases with the air without the current tiring the aquarium inhabitants.

Errors to avoid

• Pinching the intake hose: the pump works under negative pressure, becomes noisy, and wears out faster.
• Intentionally allowing the filter pads to become clogged to slow the water: the water quality will eventually suffer.
• Turning off the filter at night to calm the aquarium: the bacteria in the filter need a continuous flow of oxygenated water.
• Significantly lengthening the hoses or adding multiple elbows without thinking: the flow rate drops, but in an uncontrolled way.
• Cleaning the filter and changing a lot of water on the same day: you disrupt the biological balance twice.

The special case of Thermo versions

BioMaster Thermo models include a HeatUp heater in the filter. A flow rate that is too low or a filter that is left off for an extended period is not desirable on these versions: the water must continue to circulate so that the heat is distributed throughout the aquarium. If you reduce the flow rate, check the temperature at several points in the tank with a thermometer.

For everything else, the same principles apply: soften the water output first, adjust moderately next, and maintain the pre-filter regularly.

Frequently asked questions

How can I reduce the current of a BioMaster filter?

First, aim the output towards a glass or the surface, or install a spray bar. If that is not enough, use the adjustment provided at the connection, following the instructions for your model.

Can I restrict the filter intake?

This is not recommended. It is better to adjust the water output: an intake that is too restricted makes the pump work under poor conditions and can make it noisy.

Why is my BioMaster flow rate dropping on its own?

Most often, the pre-filter pads are clogged. Clean the pre-filtration chamber and rinse the pads in aquarium water.

What is the maximum flow rate for a BioMaster 600?

The product sheet indicates a maximum flow rate of 1,250 litres per hour, for aquariums up to 600 litres.

Should I turn off the filter at night to calm the fish?

No. The filter must run continuously so that the bacteria remain oxygenated. Soften the water output instead.

Read also

• What filtration flow rate for an aquarium?
• Understanding the filtration mechanism in an aquarium
• Where to place the pump in an aquarium?