Hmmmm....
I have lots of brass parts on my water system. Its been running for about 9 months. It handles RO mixed with tap water. I have not noticed any ill affects. I'm not a chemist or a scientist but here's what I think:
Just because the brass contains those metals shouldn't necessary mean it is going to leach them into the water to toxic levels. Example: plastics(especially silicone): Some of the individual components of the plastics are very toxic to animals. But, we don't hesitate to put them in the tank, like silicone. The silicone itself is toxic, but it is considerd the safest airline tubing. Even if they do leach a small amount of metals, it shouldn't be enough to harm the fish. Actually, I beleive Zinc is important component of plant fertilizer. All that being said, if PVC fittings are an option for what your doing I would definitely use those. They are almost leakproof, cheaper, lighter, and easier to adjust/cut. I have brass fittings on my float valves and my mixer device that mixes 80% RO/20%tap into the reservoir. These were the only fittings I could find with the right sizes and threads. Also, my tap water comes through copper pipes, and I'm sure there are brass fittings somewhere in the lines, like the water meter.
But, now you've got me thinking that if it is leaching into the tank, the effects may be cummulative. So, if anyone could confirm or deny the toxicity of brass that we be great. A link to research someone did would be great also!
Thanks!