I had a 5 reading on my nitrates, then when I tested I got a "0." The next day I tested again and the same 0 appeared. I banged the heck out of the bottle and put it in a special hobby paint shaker I use in model railroading and the bottle was shaken for five minutes - still 0.
I do have a heavily planted tank, and I'm not over the limit on bio load. I also put a much larger amount of Seachem Purigen in my Eheim canister that I suspect might be the reason - or one of them at least - why my readings are 0.
A blurb on Purigen from Seachem's site:
"Purigen® is a premium synthetic adsorbent that is unlike any other filtration product. It is not a mixture of ion exchangers or adsorbents, but a unique macro-porous synthetic polymer that removes soluble and insoluble impurities from water at a rate and capacity that exceeds all others by over 500%. Purigen® controls ammonia, nitrites and nitrates by removing nitrogenous organic waste that would otherwise release these harmful compounds. Purigen’s™ impact on trace elements is minimal. It significantly raises redox. It polishes water to unparalleled clarity. Purigen® darkens progressively as it exhausts, and is easily renewed by treating with bleach. Purigen® is designed for both marine and freshwater use.
Why It's Different
Selectivity: Purigen® is the highest capacity organic filtration resin on the market. No other products can compare to its ability to clear haziness and polish water to unparalleled clarity. Unlike other products on the market which are simple ion-exchange resins, Purigen® is specifically designed to be an organic scavenging resin. When ion-exchange resins are filled to capacity by metals and other contaminants, Purigen® has barely begun to reach its potential. Purigen® generally ignores simple elemental compounds, having an extreme affinity for nitrogenous organics. The primary source of nitrogenous compounds in an aquarium is waste. Fish, corals, even plants produce nitrogenous waste. Purigen® removes that waste faster and more completely than anything else on the market."