Horrible smell!!!

I have a 20 tall betta sorority running a 60 gallon filter with some apple snails and I am trying to get rid of ramshorns ... I also have a bristlenose and a few cherry shrimp.

After my rubberlip died the smell started ... I can't find the body now But this is bothering me like crazy. I Do regular water changes and as soon as the smell started I did an 80% change ... Which didn't help ... I don't know what to do, I am afraid if the smell keeps up my boyfriend will make me get rid of it.

I really love my Sorority and my bristlenose and my applesnails ... I already downsized tanks and only kept the ones I felt I could not go without ... My axolotl and my sorority.

Its the water specifically that smells ... It smells like ... its hard to describe ... a swamp maybe?

My filter contains charcoal. i am wondering if its the smell of dying plants

Ok That is GREAT to know Somehow I never learned that lol.

I worry there is something else I could be doing.

I just got a new piece of driftwood and my water is Nice and Tan lol ... I dont think that would cause a smell ... maybe my ramshorn taking over are causing a smell? They are quickly overstocking my tank.

My tank water stays lower than that for the snails I keep it about an inch from the top. I will try everything though.