[IslandReefer]

Just a suggestion, but a $25 TDS meter could moniter general performance of your TW filter. Keep track of the TDS and the gallons processed....then you know when to recharge (I thought the official gallons /charge is apx 50gal depending on your tap water)
I have a five stage RO/DI and Victoria water (at this time in the season) out of the tap measures only 28ppm and zero,of course out the RO/DI. If your water is any where near as good as Vics, out the tap you should be able to get serious mileage out of the TWF, thus RO/DI is ,for most of the year overkill.
Also the recharge process is very easy (search on RDO etc.)....so after using up 2 TWF, recharge and seperate the cat and anion beads (makes further recharge one step) link them and you are good to go....good luck. This is not my idea, I read it and untill I got the RO/DI as a B-day prezzy thats what I was going to do.
Gooood Reefing