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The simplest check is whether your tank water stays clear over a few days. If you're doing regular water changes and the tank gets cloudy or stays murky between cleanings, your filter probably isn't running at full capacity. Also watch for ammonia or nitrite spikes on your test kit - a working filter breaks those down. You should hear a gentle hum and feel water movement when you put your hand near the intake and output.
One thing nobody mentions is the filter media itself. Even if water's flowing through, if the sponge or cartridge is totally clogged with gunk, the filter's working way harder than it should and not actually removing much. Squeeze a bit of the media in old tank water during maintenance - if it's rock hard and black inside, that's your problem. Clean it gently (never use tap water, always tank water) and you'll be surprised how much better things get.
Here's a practical hack: stick a piece of food coloring or a tiny bit of algae wafer near the intake and watch where it goes. You should see it getting sucked toward the filter within a few seconds. If it just floats around, you've got dead zones where waste sits and rots. A working filter creates visible current patterns - not a whirlpool, but definitely noticeable movement. Pay attention to corners and behind decorations; those are where debris settles if your circulation is weak.
You'll know your filter is working when water flows steadily through the output and you see debris being pulled into the intake - check that there's gentle circulation throughout the tank without dead spots where waste settles. Listen for the normal hum or gurgle your filter usually makes; if it suddenly goes quiet or sounds labored, something's probably clogged. Also watch your water clarity and smell - cloudy water or a foul odor means the filter isn't keeping up, often because it needs cleaning or the media is too old.
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