In October I was unfortunate enough to purchase an API Freshwater Master Test Kit that had expired on the shelf 6 months prior to purchase. Needless to say I was scratching my head for months wondering why my water was so perfect!!! I stumbled across a sticky on the GAB... apparently the last 4 digits on the lot number (found on the top of each individual bottle) denote the expiry date. So if the last 4 numbers are 0310, your test kit would expire in March 2010.
So check your API kits and make sure they are still good! My kit (expired for 9 months now) is completely useless... the Ammonia, Nitrite and Nitrate always come up to 0, which for obviousreasons is NOT good! And if you can't see the expiry date when you buy it, make sure you keep the reciept!
EDIT: Read the whole thread before you chuck your test kit! Apparently there is some confusion as to whether the last 4 digits are the expiry date or the manufactured date... I'm looking into this!
Edited by kelly528, 25 January 2010 - 11:21 AM.