Many hobbyists use hang-on-back (HOB) filters to filter their tanks. These are the filters that, as the name implies, sit on the top of the rear rim of the fish tank, typically (rather than the side or the front). They’re effective, relatively inexpensive, simple to set up, and easy to maintain.
Most all HOBs work the same way. Water from the the tank is pulled up through a tube inside the tank, where it flows through a media chamber inside the filter that contains filter media of some kind. The water then exits the filter back out into the tank. Making this work requires a small motor with an impeller inside, which provides the suction power to pull water upwards, against gravity (remember, HOB filters sit on top of the tank).
Two of the most popular HOBs on the market are the Tidal by Seachem and the Fluval Aquaclear by Hagen. These are the workhorses of the HOB world. However, they are not the only HOBs on the market. There are many others.
I have used numerous Aquaclear filters for nearly 20 years. I have also used (and continue to use) Marineland’s Millenium HOB. The myth that I want to debunk is that if you don’t unplug HOB filters when doing water changes, you WILL burn up the motor. It could happen but it isn’t likely.
I am not sure where this myth started, but let me state that I do NOT unplug my HOBs when I do a water change and I have NEVER burned up a motor in any of them…in 20 plus years. I don’t use Tidals or any of the other, numerous brands of HOBs, so I can’t speak directly to them. However, since Aquaclears have long been the standard in mass market HOB filter manufacturing, most other brands have “borrowed” from Hagen’s Aquaclear design. Nonetheless, a blanket statement that not unplugging an HOB during a water change WILL burn up the motor is patently false.
Is it possible to burn up a motor in the HOB by not uplugging it during a water change? Sure. However, it’s not likely unless the water in the impeller chamber completely back siphons into the tank. That won’t happen as long as the impeller is running, and you should hear the impeller running. Yes, it is true that, if you lower the water in the tank below the intake, water will stop flowing up through the intake tube. In Aquaclears and Milleniums, some of the remaining water in the filter box itself will even flow out over the filter’s spillway. Note that I said SOME of the remaining water. If left running, once the intake of water ceases in the HOB, the impeller continues to turn. However, the impeller’s purpose is to pull water, not push it. The water pulled up through the intake has to go somewhere as the flow continues, but once the flow stops, the impeller lacks the ability to push any remaining water up and out over the spillway. So what happens? Water remains in both the impeller and filter chamber as the impeller spins. Neither of these chambers go completely dry. Burning up the motor requires that the impeller continues to run while the impeller chamber is void of water. This is very unlikely to happen as long as water remains in the impeller chamber, which it will if you leave the filter running. If you’re unsure whether the impeller is still turning with water in the impeller chamber, then unplug the filter. Absolutely nothing wrong with unplugging it if you’re uncomfortable.
Let me again state that I do NOT unplug my HOB filters when doing water changes. When I perform these water changes, the water line falls below the intakes about 80% of the time and thus water stops flowing through the filter. The motor and impellers continue to run but the impeller chambers retain water, and that water simply churns in the impeller chamber and above. The impeller chamber never runs dry. I have done this EVERY week for 20+ years and NEVER had a motor burn up.
Therefore, the claim that not unplugging the filter when doing water changes will burn up the motor is a myth. It can, under certain circumstances, but making a blanket statement that unplugging the filter causes most or all motors to burn up is simply inaccurate. However, if you feel more comfortable unplugging yours during a water change, then by all means do so. It won’t hurt anything to unplug it then restart it once you complete your water change.