I've had a 60L tank for 3 months with neon tetras and two small catfish. I do a water change every 10 days and clean the filter regularly, but it still smells weird. What's the problem? I'm using a standard Aquatlantis filter, nothing fancy.
It's probably the substrate rotting in there - you need to vacuum the bottom during your water changes, not just rinse the surface. Food scraps and fish waste pile up deep down and create anaerobic zones that really stink.
So what kind of substrate are you using, regular gravel? And how many plants do you have in the tank, like live ones or mostly plastic?
The thing is, even if you clean regularly, you can get dead zones in your filter where organic matter ferments! Standard filters build up a lot of gunk in the corners, and if the water isn't circulating everywhere properly, stuff starts rotting away quietly. Try giving the filter media a good shake in a bucket of aquarium water (never tap water!) to really get all the debris out, not just a quick rinse. It completely changes the smell after a few days!
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