Do not put a pump or powerhead in plain RO/DI water, there is no need.
By doing so you agitate the water and since it is very agressive, trying to get back to its natural "dirty" state, it will attract things out of the air at the waters surface.

I keep a 23 gallon Rubbermaid full of RO/DI for my ATO system in my garage. The container has been in use for a little over 5 years now, has never had a pump or powerhead in it and the TDS still registers 0 using a good COM-100 TDS meter. Keep it quiet or quiescent and covered and it will stay good for months or more.

Once I mix new saltwater, I then add a pump or powerhead and a heater to circulate the water and get it ready for the water change. Even then I only do this a matter of hours before using the water and any more is a waste of energy for no reason. Once the salinity, pH and temperature are matchedand stable you can't do any more for it?