30 years ago, I used ozone at a fairly low level, without any kind of monitoring, on a 100 gallon tank for 4 years. Water stayed very clear and didn't smell, despite fairly heavy bioload. How much of this was due to the ozonizer, no way to say. I did not notice any adverse effects I could attribute to ozone, and didn't smell ozone with the cabinet doors closed.
I have a Reefer 450 I started about 6 months ago. I added an ozonizer shortly after getting an Apex controller with ORP probe. I have the same Ozotech Poseidon 220 that others have mentioned. The set up is pretty simple/ basic, but works well. I maybe consider a reactor someday, but I'm not convinced I need one. One problem for me is that all these ozone reactors are bulky, and you still need to run a skimmer, which makes space a bit tight.
Anyway, here's the basics: Air pump to air dryer cannister (I just prop it up behind the tank, this is about 2/3 exhausted),
then air line to ozonizer, to ozone resistant tubing and check valve,
to port on the silencer of the skimmer, where the dry heavily ozonized air is mixed with ambient air and then enters the reaction chamber.
Although all tubing after the ozonizer is ozone resistant, the skimmer and skimmer are not specifically made for ozone, and so an degradation of these components must be watched for. The water output of the skimmer is directed onto a bag of carbon.
A flow-through carbon filter would be better but this seems to suffice. Even just hitting the rough surface of the bag will degrade residual ozone to a degree. You don't want raw ozone going straight into your tank, it's a powerful oxidizer and could be harmful to fish. Even it's breakdown products which can persist a bit longer, such as hypobromite, make many people nervous about using it.
When I first started using this ozonizer, the output was very strong, even at the number 1 setting, although this decreased somewhat after a week or so. Nonetheless, when working under the sump with the ozone on, I could still smell the sharp odor pretty strongly. I made myself a carbon-filled air filter cap for the skimmer, and most of that odor vanished. The cap consists of an 8" non-metallic sieve and #60 polyester mesh elements, purchased from the Gilson Company, enclosing a sizable quantity of GAC.
I also picked up an electronic ozone level meter, and put it about 6 feet away from the tank, on an end table. I've never seen it read anything but the lowest level there. It definitely works; put it in the sump with the carbon cap off, close the door for 1 minute, take a look and it's warning red lights all the way to the top. But one of ozone gas's many interesting properties is that it becomes more unstable and tends to decompose faster as it passes through tight spaces. Meaning just the act of passing through your closed cabinet doors, if you keep your skimmer in a sump, will remove a lot of it.
I have found that, as I continue to use ozone, less and less of it is needed to maintain the same or even gradually increasing ORP level. Right now the ozonizer is set at 4 and operates about 30 minutes a day. ORP in the sump beyond the skimmer runs 450-470, ORP in the overflow returning from the tank 360-380. Apex is programmed to turn both the air pump and ozonizer on and off based on the sump ORP probe setting and other factors. Water stays crystal clear and virtually odorless despite heavy bioload. Some say that using ozone stops skimmers from skimming; that hasn't been my observation, although I do tend to skim wet.
We sound very similar.
