I doubt a UV has too much impact on ORP.
Here's a description of what ORP is, both simplified and technical:
ORP and the Reef Aquarium
http://reefkeeping.com/issues/2003-12/rhf/feature/index.php
the simplified one:
Simplified ORP
Imagine a reef aquarium as a vast battlefield. No, more vast. Much, much more. OK, that's ORP. That is, ORP is a measure of who is winning and who is losing the battle. The battle is never won by one side or the other. As an aquarist, you do not want it to be, or else everything in the tank would be dead. In other situations, such as the purification of tap water for drinking, allowing the oxidizers to win is fine. A high enough ORP (650+ mv) can kill most bacteria in a few seconds.
On one side of this aquarium battle there are the oxidizers. They all want to get electrons, and they rip them off of the bodies of the enemy. The foot soldiers of the oxidizers are oxygen molecules (O2). Did I say the battle is vast? On one day last week, there were 342,418,226,849,748,675,496,726 of these little guys roaming my aquarium, looking for action. Some of these are paratroopers, arriving at the aquarium out of the air. Others are made in secret labs, otherwise known as photosynthetic organisms such as many corals and algae.
Unfortunately, despite their vast numbers, the oxygen molecules are not very effective fighters. In many cases, they can swarm all over the enemy and still not prevail. The true leaders of the oxidizers are far less numerous, but considerably more potent fighters. These include ozone (O3), hydrogen peroxide (H2O2), triplet oxygen (3O2), and a variety of oxygen radicals, some with such inspiring names such as superoxide radical (O2-). They also include chlorine (Cl2) and chloramine (NH2Cl). It turns out that oxygen molecules (O2) can occasionally morph into some of these better fighters (such as hydrogen peroxide), sometimes all on their own, but most frequently when they get blasted with UV light.
The oxidizers also have other types of fighters. Some are present at very low concentration, but are so sensitive to the state of the battle, that one can gauge the battle by how many of them are left standing at any given point in time. Metals, for example, such as iron (as ferric ion, Fe+++) can serve this purpose. The other oxidizers also include anions such as hypochlorite (ClO-), iodate (IO3-) and nitrate (NO3-), among a host of others.
On the other side are the reducers. The reducers all want to get rid of electrons, and they virtually throw them at the oxidizers. Many of these are organic molecules. They are not as numerous as the oxidizers, but many are much larger. Some are more than 10,000 times as large as an oxygen molecule. So they can make up for low numbers with pure brawn. That is not to say that the reducers do not have small but potent soldiers. The antioxidant vitamins, like vitamin C, for example, are small but extremely potent reducing agents. The reducers also number on their side some inorganic compounds, such as ammonia, iodide, and a really nasty fellow, sulfide.
The reducers come from fish food, metabolic waste products, the breakdown of dead organisms, and certain additives put into the aquarium (e.g., iron supplements that contain ferrous ion). The surfaces of most organisms themselves enter the fray as reducers, waiting to be oxidized by the enemy.
Interestingly, most soldiers on both sides are suicide attackers. Oxygen, ozone, and hydrogen peroxide are all destroyed when they react with a reducer. While not strictly suicidal, most organics are heavily damaged by oxidizer attacks, and are slowly degraded, eventually ending up as carbon dioxide if oxidized enough. They tend to be found in areas that the oxidizers hate; that is, in areas of low oxygen. Yet, the reducers are also sneaky, and even manage to get their hands inside cells (even finding positions in photosynthesis itself).
So Where Does ORP Fit Into All This?
ORP is a measure of the relative fighting ability of the oxidizers and the reducers. Think of the surface of the ORP electrode as a surface that these various fighters are hurling themselves against for practice. If there are lots of potent oxidizers around, and not so many reducers, ORP rises because the electrode senses more oxidizing "power" in solution. Likewise, ORP drops if it senses more reducing power in solution.
The exact value reported by an ORP electrode is, consequently, a constantly varying number that represents the ebb and flow of the battle. If you add oxidizers to the aquarium (ozone, permanganate, hydrogen peroxide, etc.) then the ORP rises. Alternatively, if you add a lot of organic molecules to the solution, or restrict the oxygen supply, the ORP drops.
What about pH? pH can impact the ORP readings in aquaria. Often, ORP goes down as pH rises. A typical aquarium ORP reading will change on the order of 59 mv/pH unit. The easiest way to understand this is to simply think of pH as a measure of hydrogen ions (H+) in solution, and to think of H+ as being on the side of the oxidizers. In reality, H+ doesn't usually oxidize things itself (though it can), but more typically it can hype up other oxidizers, like oxygen, making them much more potent. So during the course of a 24-hour day in a reef aquarium, ORP will vary as pH and O2 also vary.
Is ORP a useful measure? That is, should aquarists really care how this incredible battle is going? To some extent, yes. If the oxidizers carry the day, the ORP would rise to the point where the organic molecules that represent the bodies of organisms would be burned away. If the reducers won outright, the ORP would drop below 0 mv. In that case, there would be little oxygen left, and toxic hydrogen sulfide would rule the aquarium. In either case, the aquarium would be a disaster.
So aquarists have to hope for, and to some extent maintain, this battle in a sort of middle ground. That middle ground is typically described as being between 200 and 500 mv. Most aquarium authors have recommended a range of 300-450 mV. Why? Mostly because the ocean often has ORP in this range, and because these authors have successfully operated aquaria in this range.
HOWEVER, there is a significant potential to misunderstand cause and effect with ORP. If a crappy looking tank that is overrun by algae has a low ORP, is the low ORP the cause of the algae, or is the algae the cause of the low ORP? Or are both simply the byproduct of some other process? Does artificially raising ORP by adding an oxidizer like ozone actually improve anything? The answers are not obvious. These and other related questions will be addressed in greater detail in subsequent sections of this article that go into the scientific details surrounding ORP in aquaria.
Most reef aquarists, aside from those that use ozone and must therefore monitor ORP to prevent overdosing, use ORP to monitor if anything unusual happens in the aquarium. A sudden drop in ORP, for example, suggests that the reducers are suddenly gaining ground. That might be because a gush of organic molecules has been released from a dead organism, or because the oxygen supply is not keeping up with demand for some reason. Aquarists might use such information like an alarm suggesting the tank needs to be looked at closely. Most aquarists do not target any specific ORP value as being optimal, in part because ORP measurement is subject to considerable potential error.
So is ORP measurement and control recommended for nonscientists who also happen to be reef aquarists? My suggestion is no. There are interesting things to learn by measuring ORP, and I recommend that everyone with any interest read the following sections to better understand it and decide for themselves if it is worth doing or not. Nevertheless, I have not measured ORP in my aquarium for years, despite having the tools at hand. It is simply not very high on the list of things that one can usefully do to maintain a high quality reef aquarium, in my opinion.
