i read that just use a TDS meter and when it stops registering 0 that it's time for a filter change. whether that is the DI filter, the carbon filters or the membrane i am not sure of.
as far as RO / DI units i plan to purchase one this week. i'm going with walter from http://www.airwaterice.com/ and getting the typhoon unit which comes with a free TDS members for reefcentral.com members. i've seen cheaper units on e-bay and such but the service and all that i've received from walter thus far has been great. he's answered all my questions and recommended a unit that will do what i need with the seattle water.
I purchased a Kent Maxxima from Aquarium World back in '98 for $165(just short of theft!) I then had the pleasure of riding back to the airport(LAX) with the owner of Spectrapure. I was in LALAnd for the MACNAX. I had to tell him the reason that I bought the Kent product was that it was>$100 cheaper. Mine is rated for 24 gallons per day. I get about half that when my particulate and carbon filter get dirty (about six months.) I haven't needed to change the membrane as I don't have heavy usage. To replace all three would cost 3/4 the price of a new unit. I wish that it did have a pressure gauge.
1)Make/Model of your RO/DI unit:
AquaticReefSystems 4stage 75gph RO/DI
2)What you like most about it:
Um, great price matching. Nice, sturdy unit with highend parts. Higher flow than rated. Constant 0 TDS. Great sink adaptor with bypass. Autoshutoff valve to keep membrains wet. Nice float valve for my auto topoff...
3) What you like least about it:
No pressure valve on this model, but I don't care, so I dont dislike anything about it yet.
Thanks to all who replied with insights on this purchase, I can now post to my own thread
1) Make/Model of RO/DI unit
Air-Water-Ice Typhoon III
2) What I like most about it
No assembly required
Excellent value for my dollar
Came with a TDS meter
0 TDS even with my terrible phosphate- and copper-laden water
Rated life of one year on all consumables (dependent on water quality of regular flushing of course)
3) What I like least about it
On unit plumbing done with 1/4" non-rigid tubing, makes connections and reading the pressure gauge a little fiddly. I may replumb the pressure gauge with rigid tubing and add a temperature gauge in the process.
well i found a used Kent Maxima Hi-S RO/DI unit a few weeks ago. So far I love it. 0 TDS with the unit and that is without the DI cartridge running. I only paid 90.00 bucks for it so it was cheaper than a new unit from airwaterice.com.
my coralline algae and all seems to really have taken off since i switched to making my own salt mix and top off water.