Hi..

Should a freshwater aquarium have a smell?

Yes, like wood earth or wood moss.

Sometimes I get a "stale" smell coming from my 2.5-gallon
betta aquariums. And I sometimes get a foul smell from my
10-gallon aquarium (a dozen glo-light and neon tetras, a
pleco, a SAE, and about four ghost shrimp).

Change water. That 10g tank might be a little over stocked..

I vacuum the tank and do water changes, although probably
not as often as I should. I've also started using
distilled water, since our local tap water is very hard
and God only knows what's in it! (I won't drink the
stuff.)

Are you possibly able to collect and use (prepared!) rain
water or snow water?

--
cu
Marco