If i read this right, the tank was only a month old? If so, I would say there was absolutely no chance that digging in your gravel caused you to release something that killed your fish. Unless there was something already in there somehow? And especially seeings as how its a 200g tank we're talking about.
So in my mind that leaves a couple of other possibilities. Contamination from the Vals, or from your hands (which isnt very likely i wouldnt think). Possible air born contamination (paint, insecticide, or anything like it?) Did you do a water change before or after planting? Maybe chlorine poisoning? Either from not dechlorinating or maybe from your municipality adding an extra dose (have heard of this happening)?
Or maybe the fish got infected by something earlier, and it was just coincidence that they died just after planting?
To me, the most likely explanation is contamination somehow. If not from the plants, or your hands then i would really brainstorm about what you did around the tank or even just in the house, no matter how unlikely it seems...