I have had Tunze and other top off systems but nothing has worked better than the Genesis Storm system never failed tones of backup protection and is refills your fresh water from your ro/di unit. I have not touched it since i set it up... going to buy the renew to do auto water changes too. Just my honest opinion nothing works better than tried and tested float switches they never get film on them and in my case always work. tunze you replace too many pumps if you forget to top off your bin.
http://www.genesisreefsystems.com/