Regardless of the filter types that you use, they can and do fail. These failures come in all forms, but one of the worst is a seal failure or a hose failure on external canisters and sumps. An external filter that stops working but doesn’t leak is one thing. But that same tank losing water because of a hose, filter, or tank seal failure is something different.
HOB filters don’t tend to fail as often and, if they do, the failure is usually an impeller problem. Canister filters, while great filters, fail more often but they also have more potential points of failure. Back in 2017, I posted about my preferred position of intake tubes. In that post I provided the rationale for my intake placement. What I did not cover in that post was the possibility of a filter failure and the potential consequences of intake placement. So I’m doing that here.
If you have a canister seal failure or maybe a hose failure, water may continue to flow up through the intake inside the tank. Clearly, water may continue to exit the tank until it falls below the intake. For example, if this type of failure should occur and the top of your intake is 4″ from the bottom of the tank, then you’ll be left with only 4″ of water.
Let’s say you’re using an external canister. Now consider that a seal breaks on the pumphead of the canister. Water may flow out of the canister where it shouldn’t, but since the canister is below the tank, gravity will take over even if the canister pump shuts off for some reason. Same thing if a hose pops off at the canister or there’s a leak where the hose connects at the canister. Also, water can flow in reverse in a canister if the pump shuts off. However, many canisters contain internal check valves to prevent that from happening. On the other hand, if the outflow hose was to either come off the canister or leak at the canister, water could potentially flow backwards through the outflow. Again, because the canister is below the water surface, gravity could take over and start pulling water from the outflow pipe/tube inside the tank. In this event, the position of the outflow in your tank becomes critical. Many people place their outflows high in the tank so the water flow from the filter agitates the water surface for oxygen exchange. In those cases, water loss would be minimal if the outflow hose leaked and created back-siphon.
The very same scenario as described above could also play out in your sump. Many people employ check valves inline in their outflow (or return) hose so if the pump (submersible or external) were to fail, water can’t flow backwards. Conversely, many sump users abhor check valves and just allow extra volume in their sumps so that it can hold additional water until the return is exposed to air, at which point the back-siphon ceases. This is fine if you keep your return pretty high.
Nonetheless, to avoid loss of water from a potential hose or seal failure on canisters, you might consider the placement of your intake and possibly the return. But also take into consideration some of the factors I pointed out in that 2017 post.