Red Sea is probably the top pick of the aquarium hobbiest brands. If you don't mind spending more, Ozotech and Clearwater Tech. make units for the aquaculture trade that are reported to be better made. 100mg/hr is not enough to "OD" your 58g tank, but it's way more than you need. I expect you could run it at 10% and reap all the same benefits. For the first couple of weeks you shouldn't run more than that anyway, so you don't shock your tank. Go slow until your ORP reading stabilize in the 350-400 range. You don't want to rase ORP by more than about 20 points a day. If your tank is healthy, you may find that your ORP is already near that range. That doesn't mean that a slow dose of ozone won't help your system. Very low doses of ozone will clear the water without pushing up the ORP much at all (if any). Whether you need the probe at all is a matter of some debate. Having one is certainly better than not, but doses of 0.1-0.2mg/hr per gallon are generally safe to use without monitoring ORP. (That's 10-20mg/hr in a 100g system for the math challanged! ) So if cost is an issue, don't feel obligated to buy more than a basic O3 generator. The trick, though, is to err on the side of caution and dose very small amounts. Some believe (I agree) that a small dose 24/7 is better than using a large O3 generator and relying on a controller. Probes and electronics sometimes fail. It's just the way things are. A unit that can only produce as much ozone as you want in the tank is better than an oversized unit that will shock the system (or worse) if it decides to run 100% 24/7 because the controller glitched.