They used to be a lot more prevalent on the marine side. Some people that keep larger fish or high end goldfish run ozone to help manage waste and ammonia buildup. But it behaves very differently in fresh water, it is much less stable and decays quickly.
You need to be careful running ozone, if you can smell it (begins to be detectable by humans at around 0.05ppm), you are above the daily exposure limits where it is considered toxic (depending on the regulatory body, exposure limit is 0.06 ppm/8hr day for 5 days a week, with 0.1 ppm requiring evacution). It can be toxic to marine life at around 0.02ppm, also potentially killing off the bacteria in your bio-filter. It has a higher redox potential than chlorine. Large ozone generators are often used as part of a mold remediation program (while the building is evacuated) after the bulk of the mold has been physically removed.
When run at appropriate levels, it can be really good for helping to prevent disease, or help keep it at bay as a fish's immune system fights it off. Ozone will oxidize ammonia and organic waste, breaking it down to less harmful substances, helping keep the water clear as some of these are the organics that are difficult to filter out or cause hazy and yellow water. If feeding into a skimmer, you need a kynar venturi as it will break down many plastics.
Ozone can be broken down with UV, so using an ozone reactor before a UV unit maybe helpful. Degassing to drive off ozone in a living space would be less than ideal, especially in a new home that is fairly tight unless you have an oversized HRV/ERV with a high exchange rate (which I would recommend for anybody that is running a larger system in a newer home, especially with an open top tank).
I tend to run smaller systems where ozone wouldn't be practical. I have considered trying one of the systems created for smaller fresh water tanks just to see how it works for organics as I run nano and pico tanks and have a tendency to feed quite heavily and don't always manage to hit my water change interval every week. But I have concerns with the offgassing in my small living space and don't need an excuse to skip a water change or two. It would also be another parameter to test for. Granted, testing is with an ORP probe, so readings are quite trivial to take.
I have been working towards setting up a breeding program, which is where most of my research into ozone has come from. I may consider it if I get to a point where I am running a few large systems with a heavy bioload, periodically running ozone on each system. Or possibly running it as part of a system to condition old water for reuse. But I don't think it is very practical for my current tanks and living space.
