Some research needs to be done here-- malic and fumaric acids could be a valid alternative to citric a id;also they are not expensive..
I asked Jim Humble and here is his response:
"Take 5 drops of Miracle-Mineral-Supplement ,add 20 drops or so of any one of those acids you mention[malic and fumaric acids].Put a lid on the glass,wait 3 minutes and carefully smell."
"If chlorine smell is very strong when you take the lid off,it will probably be okay.But you don't really know how strong the chlorine dioxide really is.So I'd start out with 1 or 2 drops at first."
Jim Humble, May 8th 2008
Now if one goes to Wikipedia and do a search on FUMARIC and MALIC ACIDS
1-fumaric acid can be a substitute for tartaric acid and citric acid . Note the proportion: 1.36gram of citric acid to 0.91gram of fumaric acid