Neptune Probe ORP Double Junction

Oxidation Reduction Potential (ORP) is the measurement of water cleanliness. ORP level indirectly indicates the ability of beneficial bacteria in the aquarium to reduce contaminants like nitrates, ammonia ECT. Ultimately it is in part a measure of the electrical charge available to break down these contaminants.
Sudden changes in ORP are often reflective of organically bad things happening the aquarium. If a fish passes or too much food is available, the change in ORP can prove to be a much needed signal to investigate or take action.