I was just curious sir-I haven't looked into them yet-This was my first step-I will be setting up a salt tank this next week I believe-I going for a reef tank-I think I'm going to start off with a 40 gal breeder first-Once I get that under control-I will upgrade to my other 125 gal-Once I think I'm ready for that step-Go bigger, Most companys over rate them. say if you want a 100 gallon, i would go with one rated for 2-300 gallons.
Some companys like deltec,euroreef,ATI,H&S are pretty accurate.
Overskimming will only help you. If you notice most people that have nice tanks have large skimmers rated for more than their tanks volume.
You can get ridiculouse though.
What kind were you interested in?