Nitrites are the most common killers of aquarium fish so they are the compounds to guard against in the nitrogen cycle. Nitrites occur in the aquarium through the partial oxidation of ammonium ions. Nitrite-loving bacteria then convert the nitrite into nitrate (step 3), thereby rendering it mostly harmless.

The simplest first step in preventing nitrite build-up is to feed sparingly, making sure there are not too many animals in the tank. 

Secondly, carry out partial water changes regularly (not exceeding 20% of the total volume) with well-aged water, not tap water.

Third, make sure there are not too many total living animals in the aquarium. Many who are new to the aquarium hobby forget that even though catfish, algae eaters, and snails are “cleaner fish,” each still produces waste and adds to the total nitrite.

Instructions:

• Add 2ml of aquarium water into the test vial.
• Add 1 level spoon of NO2 Powder and swirl gently for 30 seconds.
• Allow it to stand for 3 minutes.
• Place the test vial against the color chart to obtain reading.

For freshwater aquarium, a value of not higher than 0.1 mg/L is advised.

Note:
Keep out of the reach of children! Only for aquarium hobby use.
The use of protective gloves and eye protection is recommended.
Please read the instructions carefully before using the product. 

Salifert:
Salifert is a Netherlands base company that produces top of the line test kits as well as tank additives and foods for marine and reef aquariums.
Salifert is well established and popular with hobbyist for their accurate and affordable test kits.