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I've got a 50 gallon tank and a Fluval 405 filter. I've had the filter for about 2 years and loved it but recently my tank has been pretty stinky. It just smells like a turtle tank, but with my Fluval filter I've never had odor issues, but the last couple months it's gotten more and more stinky. I changed the carbon about two weeks ago, and change it every month. What's going on? Do I need to replace the foam pads in the filter?

Like you mentioned, the foam pads may have just gotten old and clogged up with debris.if you haven't changed them for a while, go ahead and switch them out.

How often are you doing water changes, and how much per change? Any new food items that may make for bad smelling stools?

I'm refilling the tank a little every week, but when I clean it I drain about 1/3 of the water.

What do your pads and bio media look like?

Topping off the tank doesn't help too much. Go for a change and pad change. Pads are usually plastic foam and slowly break apart.