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How to Slow the Flow in Your Aquarium

Previously, we discussed the importance of filtration for fish tanks because it cleans up debris particles, grows beneficial bacteria, and helps create water movement and surface agitation for improved oxygenation. However, is it possible your aquarium filter is overly powerful and produces current that is too strong for your fish? Some fish have long and flowy fins, are small in size, or originated from slow-moving waterways and aren’t built to handle torrents of water. Perpetually fighting against fast flow can cause your fish to get whipped around the tank, start hiding in shelters, and potentially develop illnesses from the constant stress. So, if you own a betta fish, goldfish, cherry shrimp, or other slow-swimming animal, consider implementing one of these techniques to reduce the current in your aquarium.

Use a Filter with Slow Flow

The simplest way to reduce the current is to not use too much filtration in your aquarium. In their quest to have the cleanest tank possible, people sometimes install multiple filters or get oversized filters that are meant for much bigger fish tanks. Other times, newcomers to the hobby pick up an all-in-one aquarium kit and don’t realize that the default filter is too strong for bettas and other slower fish. If you see your fish struggling, don’t be afraid to downsize your filter to better accommodate their needs.

Our favorite type of filtration for gentle flow is a sponge filter with a smaller or adjustable air pump. Its coarse foam is perfect for straining debris from the water without sucking up any baby fish, and the bubbles create good surface agitation to ensure your fish get enough oxygen. Some air pumps come with adjustable flow controls to lessen the air pressure if needed, but if the pump isn’t adjustable, you can also add an air valve outside of the fish tank to reduce the amount of bubbling. If you prefer to use another type of filtration like a hang-on-back or canister filter, check to see if it has an adjustable switch or knob that allows you to modify the flow rate of the water entering the aquarium.

Nano Sponge Filter from Aquarium Co-Op

Sponges provide gentle flow that won’t harm your fish fry, bettas, and other nano fish.

Baffle the Output

There are many ways to baffle, block, or redirect the water flowing out of the filter to reduce the water pressure. If you have a canister or internal filter with an output spout inside the aquarium, try aiming the output towards the water surface or the back wall to dispel some of the water pressure. When the water “bounces” off the surface or wall, it loses kinetic energy and the current decreases. Another idea is to put a prefilter sponge on the output. The coarse sponge will dissipate most of the water’s energy while still allowing the water to enter the fish tank. If the water flow is strong enough to knock off the pre-filter sponge, try securing it by propping the pre filter sponge against a wall or sturdy aquarium decoration. Finally, some canister filters allow you to attach a spray bar to the output so that the water loses energy as it’s dispersed through a row of holes. To lessen the current even more, aim the spray bar holes toward the back wall of the aquarium.

aquarium filter output

Attach a pre-filter sponge or spray bar onto the filter output to dissipate the water pressure.

If you have a hang-on-back filter with a waterfall output, there are several filter baffle techniques that can help reduce the flow while still allowing some surface agitation. You can cut out a block of sponge that fits the width of the waterfall and stuff it into the waterfall opening. Another idea is to attach craft mesh across the waterfall opening using zip ties or string. Many people also recommend using a soap dish container with suction cups and attaching it to the aquarium wall right under the waterfall. Put some decorative marbles, foam, or even moss balls inside the soap dish to further dampen the flow.

Finally, try placing live plants, hardscape, or fish tank ornaments in front of the filter output or underneath the waterfall to help block the force of the water. More plants and decorations added throughout the rest of the aquarium will also break up and hinder the water movement in the tank. Depending on your setup, you may be able to combine several of these methods to decrease the current and give your fish the stress-free environment they need.

hang-on-back filter

Place a soap dish, plants, or decorations under the waterfall of your hang-on-back filter to lessen the flow.

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