If all you are looking for is water clarity I'd us this model and run it an hour every couple of days for crystal clear water. If you are interested in chasing some majic number(ORP) add a controller. I use this model on my 75g turned down to 25mg/hr and one hour every few days will give you unbelievably clear water. It has also been a really reliable and very easy to use unit.
It's rated up to 140g so unless you plan to upgrade the size of your tank the model 50 should do, you may just have to run it a little longer. I'm running 25mg/hr on a 75g with a 20g sump and my water is crystal clear after 20-30 minutes but I run it for a full hour. Of course if money is not a concern for $40 more you can step up to the next model. They are adjustable and you could turn it down to fit your current setup.