Hi there Dave, I have a turbo twist 18watt on my 55gal palnted and lovin every bit of it. I think these are the most advanced UV's out there. As far as oversizing on the the UV its not a big concern, just make sure you have the right flow through it to do what you want it to do. I have heard of too much exposure having some effect on the micro or macro nutirents in the tank(not sure which one) but I havent noticed any effect on my plants. That is if you dose regularly. Coralife makes some of the best products out there and you will not regret the investment made on these UV sterillizers. Keep in mind that its your choice on running it 24/7 or just when your lights are on. Having the UV bulb turning off and on constantly will shorten the life of the bulb and have to replace it sooner than running it 24/7. These bulbs are NOT cheap, about $50 cheap . I have mine running 24/7 and doing just fine! HTH
-Pete