What are your water params? If ammonia and nitrites are at zero, then there is no reason to go with a canister. Nothing wrong with overfilration but most of the time it is not needed. I'm with ewurm though if you need more biological filtation go with a wet/dry sump you can build you own for alot cheaper than you can buy a canister.
Water changes are king... Overfiltration will not necessarily reduce nitrates. Nitrates are going to build up in the tank regardless how much filtration you have. I suppose if you set up mechanical filtration in such a way that you are constantly removing waste materials before they turn into ammonia by rinsing or changing out filter media then you can reduce the ammount of nitrates that eventually accumulate.
My advice to anyone getting into the hobby is set up your tank(s) in such a way that water changes are as convenient as possible and then *do them*. All of my tanks rely almost entirely on bio-filtration and water changes. I don't think I would buy a product that reduces nitrates. Water changes are removing more than nitrates from the system and providing other benefits as well.
