What I've been planning on doing is masses of small water changes in an automated fashion. While not removing the need to change water, it may remove the hastle you're talking about, and shouldn't cost more than around 10 or so dollars a month... I had in mind having pre-made, circulating, salt-water ready to go (in the 100 - 200g range), and doing a gallon or more, per day day, which would bring me to above my 10% change... essentially pump out to staging pair of containers with measured float switches.

Here's the process:

* Pump to staging A from storage
* When high float switch in staging A triggers to say water is in container at level, shut down ATO, and pump water out of tank into staging B
* When staging B triggers it's 'high' float switch, start to pump water from staging A into tank
-> If *tank* 'high' float switch is triggered, or salinity is raised above a certain point, or below a certain point, *alarm* and wait for user (continue with ATO disabled, and cease all operations).
* Once pumping has been completed from A, and 'low' float switch has been triggered within the staging tank, stop pump, and re-enable ATO
* Then pump water from B to waste

This'd also allow me to have a large container of mixed water somewhere far from my tank, and only needing two lines running to my tank. Similar to what Dipan has for water storage / salt water storage. The reason i bring this up is that I've given a lot of thought to yearly costs for water / replacement (assuming that you ignore the environment, and assuming you're doing a 10% change per week, in a magical perfect world..). What i figured was at around 200G, I'd be paying give or take 4 dollars for the water (10 if you include waste from your R/O, I use mine to water the garden, and for other things, so I don't), and around 50 or so for the salt, depending on brand. If I then ran a Evo 750+ in the source tank, i'd be spending around 3 - 5 dollars per year for running the powerhead. If I used 10.5G per week (1.5G per day), my replenishment rate would be around 200G every (just under) 20 weeks, giving me a yearly cost of around 12 for the water, 150 for the salt, and lets say 10 for the electricity, so safely under 200 dollars. Granted the capital outlay for this would be in the region of around at least a few hundred dollars, but it should remove any need to add supplements to the tank, and hopefully keep most of the measurable thresholds within any reasonable range... ph/etc... I do however wonder what this would do to 2 part dosing...

Also, while your idea sounds pretty cool, I'm not aware of any reasonably priced UV lighting that would be able to actively remove all the critters within a reasonable price range, without having to circulate it through the UV system... although I'd have to look into that more, it's certainly an interesting idea, although would you not be worried about starting a potential cycle in your vat of water each time you worked on it? Without livestock, and a balanced system, some of the critters may just die, and leech more stuff back into the water. That said, some of the chemicals you'd have to add back may far exceed the above mentioned costs... which could probably be halved if you just did less frequent water changes. Great idea thou!