What chemicals can I use to lower pH levels of DI Water and Soft Water

First, for soft water, it is normally just tap water that has had the poorly soluble calcium ions replaced with chloride ions which are very soluble. For this, HCl, muriatic acid ⇦this on eBay or Amazon [affil links] is probably the cheapest. Use it in a good grade and add it in a dilute form to avoid overshooting the desired point.

For DI water, IF you have to lower the pH, find an acid that is compatible with the chemistry that you are using it with. This might be sulfuric or HCl or sulfamic or----.

I think that the smell and cloudiness is due to bacterial growth build up. Years ago we used formaldehyde (now banned) to slow this down. As long as you move from ph above 7 to < 7 you will alway experiaence this (I think).

Don't know a whole lot about your situation, but it's not good to add chemicals to the purified water tanks. The recirculation of the water should maintain the pH. If recirc doesn't maintain the pH, your filters/beds could be bad.

There is phosphoric acid. It is quite non-volatile and non-hazardous.

You might also look at sodium bisulphate. An easily handleable solid, it used to be included in kids chemistry sets as a "safe sulfuric acid."