Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
'A bit musty/bad' sounds like Cyanobacteria, Blue Green Algae.
All of my tanks also smell more like good garden soil. Lots of plants, microorganisms, healthy system.
It may well be that when something dies in a non-planted tank it lingers, not getting decomposed quite so fast, so starts to smell. AC removes a lot of organic molecules, including molecules that contribute to the bad smell.
In a planted tank there is a greater reservoir of decomposer organisms so when something dies it is decomposed quicker, and the plants remove any of the small molecules (whatever fits into their cells) so quickly that you do not smell it.