Thanks Chefkeith, sorry it took me so long to get back on this.
Background on what RO water is:
RO water is water purified by reverse osmosis. The RO process removes over 99% of everything, including ALL the hardness and buffers. Thats what I want it to do. So, being that the water is nearly pure the ph should be almost exactly 7.0, KH=0; which is what it comes out of the filter at.
So, even very small amounts of anything will greatly affect the ph of RO water. My reservoir had a constant amount of water in it, so I believe the tub started to leach into my water. But, I don't have the ability to test for what could heave been leached to know for sure.
Reason I use RO: My tap water is gross here and has a KH of about 18! So, in my tanks with CO2 I use 75% RO and 25% carbon filtered tap water to achieve a KH of about 5. Perfect IMO.
About Rubbermaid tubs:
I forgot they made food grade tubs. I think the food grade ones would work great! The ones I saw on the link were the cheap ones we find locally, which is what I used. I have done a little research recently and come to this conclusion. The tubs should be just fine for temporary use, a couple weeks to a month or two. The longer they sit full of water, the more they start to leach. I talked to a guy that used a kiddie pool and rubbermaid tubs and ended up losing a lot of fish before he figured out the plastic was the issue. They were in use for over almost a year before he started to see fish death, poor plant growth much before that. So, in a reservior situation one would assume the tub would sit full of water for extended periods of time, making the plastic an issue.
Just what I think, and info I have gathered from my own experiences. BUT, if it works for you, go ahead. I am not going to chance it, though.