Demystifying ORP

Demystifying ORP with a Minimal Amount of Chemistry
by ACoralNut

I have seen the popularity of salt water aquariums explode over the past decade, and it seems that more and more people are up to the challenge of keeping a salt water aquarium. Congratulations to all who are successful in this hobby - although you can make a salt water aquarium as easy or as difficult as you want, it is still a challenge to recreate an ecosystem as complex as the ocean in what literally sums to a bucket in your living room.

A number of ‘complex’ instruments were introduced to the hobby in the 60's. While these instruments may be technologically complex, the theory that operates behind most of them are quite simple. Even more importantly however, is what these instruments show about your tank. These ‘complex’ instruments can in fact make care of your tank simpler.

One of the most valuable instruments available is the ORP meter. No single device can be placed in your tank to tell you if everything is perfect, but an ORP probe is a good indicator of how well your tank is operating, and how it can handle stress. First, let’s look at ORP tells us about our tank’s water.

The root of all problems in our tanks is, surprisingly, our fish. This may seem ridiculous at first, but think about it: your fish are literally swimming in their own toilet. Imagine how happy you would be, confined to a bathtub full of waste? When fish release their wastes into the water, it deteriorates the quality significantly. These wastes are toxic and can kill.

Wastes however, are not bad under certain circumstances. When we talk about the toxic effects of a compound, we look at both its concentration and the duration of exposure. If we are exposed to a massive amount of toxins for a brief period we can be harmed. If it only happens once in our lives however, we probably recover and don’t suffer any long term effects. Now, think of the following; what if you are exposed to minute amounts of a toxin each and every day of your life? Even if the dose you get isn’t enough to kill you, or even measure, the effects build up over your life and you will probably suffer ill health effects. Of course, long term exposure to high amounts of toxins is probably fatal. Brief exposure to minimal amounts of toxins is what we more or less want in our lives.

Look at it another way; let’s say an oil tanker sinks off the coast of Spain. There’s an initial flood of oil that wipes out a lot of costal life, but it is eventually cleaned up Life recovers and populations re-establish themselves. These high intensity but low frequency events don’t make long term effects if they are handled. Think however now, of the tanker on the ocean’s floor, slowly leaky tiny tiny amounts of oil over several hundred years. Sea life slowly exposed to this long term, low intensity poison is likely to show negative health effects.

So fish are constantly swimming in their own toxic toilet. Fortunately for us, the wastes from fish are processed from their toxic state into benign compounds. When fish waste is broken down into safer compounds, the original toxins are undergoing a process called oxidization. Almost all processes that occur to destroy fish wastes are oxidization processes. Fish wastes are oxidized so that your delicate fish aren’t exposed to the potentially toxic compounds they give off. As a side note, the opposite of oxidization processes is called reduction. In fact, whenever a waste is broken down both processes operate. One compound (the fish waste) is oxidized and something else is reduced to allow this to happen.

For the first time in this article, we’ll look at what ORP stands for. ORP stands for Oxidization / Reduction Potential. What does that mean? We have discussed that oxidization is the process by which fish wastes are processed to become less toxic. What is a tank’s potential to oxidize wastes?

Let’s take an imaginary tank and put a fish in it. Let’s say this fish introduces 1ml of waste in a day. If the tank has the potential to totally process all that waste, it's good - you won't have ammonia building up in the water. The tank has the potential to oxidize 1ml of fish waste. Let’s add another fish. Now we have 2ml of fish waste being added to the tank per day. If our tank can still process that much waste you can add more fish. This tank has the potential to oxidize at least 2ml of fish waste per day.

But sooner or later, you will reach the limit of the tank to process this waste. Let’s put a total of five fish into the tank. We thus have 5ml of waste being added per day to the tank. If the tank can only process 4ml of fish waste per day, the ammonia will start to build up and the delicate sea life will die. This tank only as the potential to oxidize 4ml of fish wastes per day, and 1ml of waste will build up each day.

So, ORP is the potential of your tank to oxidize fish wasted. The higher your potential, the more wastes your tank can deal with. High ORP levels are good, because your tanks is theoretically processing all the fish wastes present in the tank. Low values mean that there is too much waste for your tank to deal with, and negative values show a strongly reducing environment - the opposite of what we want in our tanks.

ORP has a range of values that are commonly measured in the tanks. Positive values show that the tank can oxidize at least some wastes. Smaller values suggest that less waste can be processed relative to high values. Negative values tell us that things are actually undergoing the opposite of oxidization and are being reduced. Zero is an environment that is neither reducing or oxidizing. One may ask why zero is not a safe value. The answer to this is that biological entities have chemical reactions constantly going on in their bodies. How long can you go holding your breath? A minute? Half a minute? Without an oxidizing agent to drive some of the chemical reactions in your body, it’s not much time before your body shuts down entirely. (Note: I am not responsible for injury or damage you may sustain from holding your breath)

One may ask what values we want to see in our tanks. This is, in fact, the wrong question to ask. Anyone who measures ORP in their tank knows that it can vary wildly from day to night. What is important is a steady cycle. If you see a different ORP just before the lights come on, at midday, just before the lights go out, and in the middle of the night, that is the sign of a healthy tank.

What makes this cyle of values? Fish produce more wastes when they are active. Generally, tanks have a drop in ORP during the day as your fish come out and become active. Activity increases an increase (an increase relative to sleeping) in metabolic processes. The wastes these processes produce are oxidized, ‘using up’ some of the ORP. During the day photosynthetic plants add oxygen to tanks and increase ORP - so we might see slight ORP rises during the day in heavily planted aquariums. A healthy tank goes between about a potential of 380 at night to 400 during the day - above 435 is sometimes harmful, as it can theoretically oxidize fish flesh right on the bone! This is why ozonizers, which raise ORP, can kill fish if overused. (Note: It has been observed by myself and several other aquarists that this may not be true, but I don’t intend to steer you in a direction that may harm your livestock. One individual has told me that adding ozone directly to the tank via an air bubbler and diffusor stone has never harmed his livestock. I have often overdosed my tank with ozone, with no ill effects at all) An OK tank is about 350night/390day. Values below that show that wastes cannot be properly handled by the bacteria in the tank. Negative values mean certain doom.

Remember, it is a steady cycle that is important; exact values are not what one should be aiming for. Any deviation from a cycle that has been consistent for 3 weeks should tell you that something has changed. A steady decline could come from the death of a fish, or excessive feeding. Low numbers but a consistent cycle could easily result from overstocking the aquarium, as could low values during the day and normal numbers during the night. Cleaning the detritus from your live rock could give a 20 point boost at all times. Note that the detritus in your tank can only be oxidized so far - once it has lost its reactivity it is more or less inert, and simply builds up. This is similar to why there is so much ‘dust’ in our terrestrial world - it’s inert, so nothing reacts to it and it builds up. Actively decomposing detritus however, will lower your ORP. The objective is to have a steady cycle with high numbers. If your ORP cycle changes, always ask yourself “What would cause the ORP to change in this direction? Should I be concerned about stocking, deaths, overfeeding?”

Not only oxygen can oxidize materials in an aquarium. Many different materials can contribute to a healthy ORP value. Household bleach has an extremely high ORP - but don’t go pouring that into your tank to clean up the detritus! Other chemical reactions can occur when a chemical is added to the tank. Hydrogen peroxide can be added to increase the ORP in a tank in the short term. Dosing any ORP increasing compound is no substitute for a clean, healthy, properly stocked tank.

Now, let’s not think that ORP is the be all and end all of water sampling. Remember, it is just another tool to make the hobby easier. I’ve seen a few aquarists show off their tank to me and tell me their ORP is great, when their fish are crawling with external parasites or have HHLLE. Again, ORP is a good overall indicator of tank health. If your ORP is high and there is nothing else to indicate your tank is in poor shape, you are probably doing a very good job of caring for your tank. If something else is noticeably amiss, it should be addressed.

When to add fish, when to clean your tank and the effects of short and long term toxin exposure can all be shown to you by measuring the ORP in your tank. As all measuring instruments vary in accuracy and precision, so always ask someone who has purchased one if they are happy with their product. Toxicological literature comparing the effects of low intensity / high frequency toxin exposure to the effects of high intensity / low frequency toxin exposure is available on the internet. Search terms to try are dose, dose response, periodicity, frequency, intensity, LD50, and toxin.

About the author:

ACoralNut has kept a few salt water tanks over the past decade in Canada. Where everything costs three times as much. But he’s not bitter. Not bitter at all...