Nitrite (NO2) is produced from the breakdown of ammonia in the filter. This is highly toxic to fish, but is usually removed from the water by the bacteria which live inside the filter. In new ponds and aquariums, or if the filter bacteria are damaged, it is easy for the nitrite level to become toxic. The only way of knowing what the nitrite level is in the water, is by testing it with a kit. The Colombo Nitrite Pond Test Kit contains 40 tests.