"Altum" wrote in news:1140286709.246608.305140
@g14g2000cwa.googlegroups.com:

the smell of a healthy, mature filter. Am I the only one who "tests"
water chemistry by sniffing it?

Nope, you are not alone. With more than 260 tanks to look after, small is
often my first clue to a new potential problem when I walk into the fish
room in the morning. I will often follow my nose to the trouble spot.

Ammonium, nitrite, and nitrate all have their own distinctive smells.