I would avoid that skimmer you mentioned. Look at a Reef Octopus for something inexpensive yet still works well. The Coralife is inexpensive but doesn't work well.
BTW, I spent 7 years making saturday my "go to the store for 20G of RO water". I was stubborn and didn't want to buy an RO/DI unit for some reason.. after getting one I have no idea why I waited so long but it took having problems with the store water (old filters/high silicates causing major problems) that made me finally buy my own unit. Worth every penny.
My experiences exactly. ^^^ You can't count on the vast majority of LFSs to keep track of their water quality. I know of several instances of stores that, while they do indeed keep track of quality, they ignore it when it rises and lie about it. I doubt many would go that far, but without working there, you'd never know. It is worth the peace of mind to get your own unit that you have complete control over.