ORP (redox potential)

ORP measures the oxidizing power of water in millivolts. A high ORP indicates clean, well-oxygenated water. A low ORP indicates accumulation of organic substances.

Detailed explanation

ORP is a little-known but very useful parameter as a general indicator of water quality. It is measured with an electronic probe in millivolts (mV).

An ORP between 300 and 450 mV indicates a healthy and clean aquarium. Below 200 mV the water has too much decaying organic matter. Above 450 mV the water is very oxidizing (happens with ozone).

ORP doesn't say what the problem is. It just says something is wrong. A sudden drop in ORP after heavy feeding is normal. A progressive decline over days indicates accumulation of debris, dirty filter, hidden dead fish.

Ozone (O3) dramatically increases ORP. Some aquarists use an ozonizer connected to an ORP controller that doses ozone automatically to keep the ORP in range. But ozone must be used with caution: in excess it kills everything.

For freshwater aquariums ORP is less used but the principle is the same. A healthy planted aquarium has ORP around 250-350 mV.

Reference values by aquarium type

How it is measured

ORP probe connected to a controller (Neptune Apex, GHL). There are no drop tests for ORP. The probe must be calibrated periodically with reference solution.

What happens when it is too low

Water loaded with organic matter. Ammonia risk. Filtration to be strengthened. Look for causes (dead fish, excess food, clogged filter).

What happens when it is too high

If ozone is used: risk of oxidation of the gills. Without ozone a high ORP is rarely a problem.

Related terms