* Other vendors? Other options? Overkill?[
IMO all RO units are the same, its just different bells and whistles and filters they put in, I'd go for cheap, but BRS is usually pretty good on prices so you shoulid be good with that. Plus they supposedly got great customer support in case you have questions.
Options, a pressure gauge is nice to verify your water pressure as well as see if there's a drop through the unit (clogged filter), TDS meter is nice, I see that on as well. Booster pumps are great, but I see your pressure is 70PSI so that's not necessary. I would suggest getting another membrane and hooking it up to nearly double your production and halve your waste water (I think BRS sells a kit for that), you have sufficient pressure so should be good. Other than that it looks like a decent unit. I wouldn't worry about chloramines or buying one that's "ready for it" two carbon blocks will more than take care of any of that. Lastly an auto-topoff switch might be great so you don't start making water and forget about it, and flood the basement.... a couple few tens of times over years of reefkeeping
As to overkill, don't get filters that have "special qualities", IMO they're a waste of money. Also stay away from filters which are too small, or you'll be swapping them out much quicker than the recommended switch dates because they'll clog on you (pressure gauge should show this)
* I went with the six stage because of a claim that the dual DI filters last a lot longer than with the single DI filter. Savings in the long run for not too much more upfront cost. Is this even true or important?
Well 2 DI containers will last twice as long as 1
It won't save you anything in the long run though, however it is nice since they're usually packed with the color changing stuff such that you can see when they get exhausted, and the first one will go first, so once it's gone the second one kicks in and it starts going, well hopefully before that other one goes you can get spot it and change it out. If it's not too much more money then it's a decent option, if it is quite a bit more, you can usually get a secondary kit to fill the need later if you feel it.
* I have ~70lbs water pressure. What is going to be waste water lose? Does water pressure vary much? The plumber I asked wasn't sure.
Most RO's give a 4:1 waste:good ratio, if it's really cold, or your pressure is low, then that can go up to 5:1 or more. As mentioned two RO filters in series will reduce that waste water down to 2:1 or so if you really care that much. Your pressure is great, and it won't really vary much anywhere in your house, since it's on the supply side (street) that's providing it, and not a pump or something
* I currently mix my WC water in a 5G bucket with a heater and a tiny pump. Once I go RO/DI should I really start mixing water in larger batches (50G brute bucket?) even if I usually don't need that much at any given time?
How much of a water change are you planning on doing? I usually keep a 30g can of fresh water on stand by simple for top off, but most don't have the ability to keep big containers of water around. If possible I'd keep a good sized container of RO water ready, and if you do 5g changes, no problem just take from that. If you do larger changes, get the appropriate sized cans.
* I have been reading about ATO. It seems that hooking a single float switch ATO directly to the RO/DI filter is a bad idea because of small run cycle length. RO filters want to be run for a couple of gallons minimum. So an ATO reservoir with it's own ATO to refill the resivoir? urk. What's normally done? For ATO setups which use two float switches (bottom/top) I think I would need a bigger sump to avoid salinity changes (though probably still better than my current "lug-a-jug" method).
see above.
As to the salinity changes I wouldn't worry terribly much about it, unless you are evaporating a large fraction of your water (in which case you already have the salinity change). If you want, and have the ability simply hook some sort of ATO from your reservior of fresh stuff, and hook it too a timer to turn on a few times throughout the day.
o The drinking water setup for the filter includes a 3G pressurized tank. Can I run the DI filters off of the drinking water tank to accomplish a direct connect to the ATO? I haven't heard that DI filters care about short run cycle.
The pressurized tank is basically for drinking water, I don't like using them, I think the water is fine personally, and it's one more thing to go wrong
While you could run from the pressurized can to DI, you do run into issues because as your can empties it'll pop on again to refill the canister so essentially you're still hooking it up to your RO unit. Most importantly though, I wouldn't advise anyone to hook up any sort of ATO to the supply side of things (i.e. the faucet/RO unit/etc) because if something ever goes wrong with a float switch you'll have access to an infinite amount of water that will keep coming and not care what it kills.
* It sure would be nice if someone combined an ATO/WC system for cheaper than this setup Genesis Reef Systems.
If you're a tiny bit handy and have the ability to have a "reservoir" as mentioned above... http://www.aquahub.com/store/index.html
* I want to eventually get a controller/doser for ALK/Ca. For the ATO in particular what should I know now to avoid having to rebuy a different ATO later.
You'll still need to top off freshwater in addition to Alk/Ca, I mean technically you could get around it by diluting your 2 part solution but you'll need to know how much evaporation you have (which changes all the time), and is just not worth the headache. When you get an ATO set up (whichever route you go), you'll need separate ALK/Ca dosers... unless you went with one of those multi-dose units like the bubblemagus one which I believe has 3 pumps hooked to it.
