Here's how to figure it out:
* Check the instructions: Most filter cartridges will have arrows or diagrams on them indicating which way they should be installed.
* Look for a flow direction: Some filters will have a "flow" direction indicated, often with an arrow or similar symbol. The water should flow in the direction of the arrow.
* Consider the filter type:
* Sponge filters: These are often placed vertically, and water is drawn into the bottom and flows up through the sponge.
* Canister filters: These usually have a specific direction for the cartridge to be placed within the canister itself, so consult your filter's manual.
* Hang-on filters: These often have a filter cartridge that sits in a basket or holder. The water should flow through the cartridge in a specific direction.
* Common sense: If you're still unsure, think about how water would naturally flow through the filter. It usually flows from the bottom up or from one end to the other.
Important Note: Installing the filter cartridge the wrong way can reduce its effectiveness and potentially harm your fish. Always check the instructions and follow them carefully.