What single thing...

[asylumdown]

I'm going to second Brad's plumbing the R/O in to the top off. The only reservoirs I need to worry about being filled now are my dosing reservoirs, and they're 4 gallons each so they last a pretty long time (though, less and less time recently. stupid sticks). In the last tank, going out of town meant hauling out a 20 gallon bin, filling it up with water in the middle of our living room, and re-plumbing the entire top off system so it had a bigger reservoir. It took over an hour to test it each time. My 'out of town routine' now consists of directing the skimmer drain hose to the sewer drain under the tank and opening the gate on the auto-feeders a little.

But also the internet and forums. I was groping in the dark when I started this. Books are helpful, but there's so many obscure "what about 'this' in 'that' application?" questions that you'll never be able to find a book that covers all the questions or problems you'll face. Without being able to ask people and get real and immediate feedback, I probably would have killed way more things than I have.