if your tank has a funny smell then here are some suggestions as to what it could be
Fishy smell - strong pungent smell that resembles a dead fish that is not freshly caught. Smell is typical of ammonia or nitrite, but can also be caused by dirty gravel, filter and water. Solution = partial water change with a gravel vacuum and new activated carbon. You may also see a blue green algae or a white cloudy tank outbreak if let go

Mildewy smell. I have only had this smell occur in a planted tank that was starting to crash. I have not been able to find a lot of information on this smell, but I have a strong suspicion it is the first indicator of plants starting to stagnate and rot. You may also see a blue green algae outbreak.

Rotten egg smell - This is hydrogen sulfide buildup do to an anaerobic gravel bed. Very carefully gravel vacuum as release of hydrogen sulfide bubbles can be detrimental to your fish. May also see nitrite levels when tested for as denitrification takes place in anaerobic conditions.

Earthy smell like rich compost - this is a good smell that signifies that your biological filter is breaking down wastes and functioning properly. This is how your water should smell always.

hmm... is there any invention of a smell detector?
then we can add one more test kit to our test kit box.