Cobalt MJ900 Powerhead Review

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Size Dimensions Tank Size Noise Level Longevity Max Height/Head
230 GPH 5.5″ x 3″ x 2″ 10-35 Gallons Very Low 7+ Years 46″

The MJ900 Isa high quality version of the Maxi-Jet, or more precisely the older version. It retains the use of metal components and is produced in Italy rather than China.

The over all lifetime, reliability and multiple functions this powerhead provides makes it well worth the price.


Setting up the MJ900 is a quick and easy process. The main body of the powerhead comes fully assembled, leaving the choice of grate, flow attachment and mounting device up to you.

The mounting devices to choose from are the triple suction cup mount and the hanging bracket mount. The suction cup is generally much quieter and can be placed anywhere in the tank. This is my preferred method, as I ended up using it to pump water.

The hanging bracket keeps the powerhead in place near the top of the aquarium. This is good when you are using the air injection feature. This is done by connecting the provided air line tubing to the output of the powerhead. Remember: the air line tube must be kept above the powerhead. In the event of power failure this prevents a reverse siphon. If the air tubing goes below the pump it will pull water out of the tank until both ends of the airline tube are above water.

The two grates to choose from will create either a sponge filter or under gravel filter. The sponge filter is made simply by placing a piece of filter floss or sock around the shorter grate. This removes the larger debris in the water, and must be rinsed out regularly.

To use the powerhead as an under gravel filter you will need the longer, thin grate. This pulls water down into the gravel, trapping debris and promoting beneficial bacteria growth. The trapped debris can quickly be removed by either netting out the surrounding gravel or siphoning the water around the powerhead.

The output of the MJ900 is a simple tube. This creates a fairly harsh output of water, more suited to fast swimming fish or to use with 1/2″ ID tubing.

As a final note the cord is 6′ long, which is actually a bit too much cord for me considering the size of the tanks.


The MJ900 uses an impeller rather than a propeller, which eliminates a lot of issues many other powerheads have, such as running in reverse or loud start ups. Additionally the high quality design of the impeller means it will not be damaged easily and runs nearly silent.

The motor/body of the powerhead comes apart from the nozzle and grate, which exposes the impeller and impellers mount. these should both be cleaned using soft materials, to ensure that the magnetic components are not damaged. Damaging these will cause irregular spinning, damaging the impeller and causing a fair amount of noise. Cotton swabs are always my go to for cleaning these two pieces.

The grate itself can be scrubbed clean. It is fairly tough and can even stand up to sand paper, so you don’t need to worry as much as cheaper models.

Special Note: The warranty on the MJ900 does not protect against calcium build up, which frequently happens in the motor. This mostly occurs in reef aquariums, where calcium concentrations are much higher. To prevent these damages Cobalt recommends soaking the powerhead in distilled vinegar for up to 20 minutes before scrubbing the motor clean. Rinse the powerhead off with water before returning it to the tank. This should be done every 2 months.

Flow & Adjustments

The flow rate on this pump is not adjustable while the direction is fully adjustable. This is done using the venturi adapter, which looks a bit like a vacuum cleaner attachment. This slides onto the output of the powerhead and can be directed anyway you choose.

For owners who house slower swimming fish such as bettas you may want to diffuse the waterflow. This is done by aiming the powerhead at the wall of the tank. This helps distribute the flow and makes the current overall much more gentle for your fish.

Warranty & Lifetime

Cobalt provides a 3 year warranty which covers most forms of damages. They do exclude human caused damages, with the exception of shipping, as well as calcium damages. To prevent this please refer to the end of the assembly section.

To use the warranty, or contact customer support in general, you will need to call Cobalt at 803-591-9500.

The lifetime is where the higher quality design really shines. Most users of Cobalt MJ powerheads will see well over 7 years of lifetime. This is far greater than most small powerheads, excluding Tunze. The price however is well below Tunze and not much higher than cheaper brands such as Marinelands Maxi-jet or any SunSun

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