Demineralized water will absorb carbon dioxide from the air. You will see the pH drop when this occurs. For this reason, demineralized water should be sealed from exposure to air.

Stainless steel has the potential to stain if it has been exposed to carbon steel. If you use a carbon steel wire brush on stainless steel, the stainless steel will show rust stains where the carbon steel brush has contacted the stainless steel.
Under certain conditions, rust spots may appear on machined parts. This is actually corrosion of foreign particles from the tool steel, not the parent metal. Sometimes the crevice at the embedded tool steel particle or its corrosion products may cause attack of the part itself.
Likewise, small particles of iron-containing shop dirt may stick to the stainless part surface. Although the metal may appear shiny in the as-machined condition, the invisible particles of free iron can cause rusting on the surface after exposure to the atmosphere.
Cleaning
Grease, coolant or other shop debris must be thoroughly cleaned from the surface to obtain the best possible corrosion resistance. Machining chips or other shop dirt can be wiped carefully off the part. A commercial degreaser or cleanser may be used to clean off machining oils or coolants. Foreign matter such as thermal oxides may have to be removed by grinding, or by methods such as acid pickling.
Occasionally, a machine operator might skip the cleaning, falsely assuming that by immersing a grease-laden part in an acid bath, both cleaning and passivating will take place at the same time. That doesn't happen. Instead, the contaminating grease reacts with the acid to form gas bubbles. These bubbles collect on the surface of the workpiece and interfere with passivation.
Even worse, contamination of the passivating solution, sometimes with high levels of chlorides, can cause "flash attack." Instead of obtaining the desired oxide film with a shiny, clean, corrosion-resisting surface, the flash attack produces a heavily etched or darkened surface. This is a deterioration of the very surface that passivation is designed to optimize.
http://www.cartech.com/techarticles.aspx?id=1456

This information should be well known to a good tank fabricator.
 

But would CO2 cause the drop in resistivity so quickly?  It's literally immediate as the water enters the tank.

I'm looking into pickling the tank.  After that, I understand that perhaps the gray PVC might be a good alternative?  The tank is in an enclosure so it will not be exposed to direct sunlight.  Will rubber gaskets around the metal pipe (to prevent leaking) affect the water quality?

Onwards,

Matt

I would suspect that you would have some water soluble salts or residue in the bottom of the tank. If water evaporated from the tank and left a residue, that residue is water soluble.

You have to have the tank thoroughly cleaned out prior to use.

Anything that is water soluble will effect the water quality.

What are you talking about, gray PVC?

What are you using this water for?

Hi, demin water of that quality should not drop just like that. If storing the volume the tank should be sealed, on some applications a Nitrogen blanket is used above the volume of the treated water.If recirculating prior to use you may want to install a polishing cylinder.