The Fluval FX4 and FX6 are high performance aquarium filters that will pump the tank water to your filtration system of the canister and them pump it back out to the fish tank. If the device is pumping out a lot of micro air bubbles instead, then it is most likely not working efficiently.
To see how you can stop the fine air bubbles from being pumped out of your FX4 or FX6, you can take a look at this troubleshooting article from Aquarium Blueprints.
1. Before we begin, it should be noted that the Fluval FX4 and FX6 will automatically remove any air inside of its canister shortly after you plug it in. The device will also remove air every 12 hours to prevent any buildup.
So, if you are still seeing micro air bubbles from the output nozzle, then it indicates that something is wrong.
2. Air can get in the filter through the output nozzle when the filter is priming. In order to prevent this from happening, make sure you position the output nozzle to above the water line.
3. You can also try unplugging the device from the power outlet, drain the water from the canister and then fill it up again.
Fluval recommends that you fill the FX4 with 1.6 gallons (or 6 liters) of tank water before starting the priming process. For the FX6, you need to add 2 gallons (or 8 liters) of tank water to the canister.
Once you refill the canister with water, you can plug it back in the power outlet.
If you are experiencing some difficulty in trying to prime water to these products, then we recommend that you check out this guide.
4. Moreover, you should make sure you have the canister filter properly set up. Otherwise, you may experience starting or pumping problems.
To find out how you can fix these particular issues, you can take a look at this link.
4. If you are still seeing your filter constantly produce micro bubbles, then the next step we recommend taking is to make sure that the hosing that is securely connected to the “IN” and “OUT” valve as any openings may lead to air getting into the canister.
5. Avoid putting an air stone or bubbler next to the filter intake. If it is too close, then the air created by these may be sucked into the FX4 or FX6 canister.
If you are using an air stone or bubbler, then we recommend placing it on the far opposite end of the filter intake. By doing so, you should be able to create efficient water flow all over your tank water column.
6. It is also important to note that new media added to the filter may produce air in the beginning. To prevent micro bubbles, you may want to consider pre-soaking the filter media to get it water-logged.
Furthermore, we suggest that you avoid using big bio balls as these may cause a lot of bubbles to appear in the canister.
7. There is a possibility that the rubber connector may need to be replaced. These parts tend to harden over time, which may let air into the filter.
If you need help with finding the right rubber connector for your FX4 or FX5, then you can check out this post.