Not meaning to contradict--just trying to share my experience, but I use RO water for all my mantids, and don't have any problems. However, I was told not to use purely distilled/RO water for my tree frogs, so I actually make a mixture of half RO/half spring water for their soaking pond.
Thanks for writing, feel free to correct anything I say wrong I'd rather know something correctly than incorrectly.
Although, you are simply responding with the same data I pointed out, including mixing the water.
Sure while the straight RO water seems like it doesn't affect mantids, it is known to affect other pets (amphibians and fish as described/linked to above). That is a sign of potential health issues for anything receiving the water theoretically, including mantids. Of course without any obvious issues or distress we do not know the full range of possible issues (or lack thereof), in which case a scientific study would be the only way to know for mantids (and we know how few those are).
Various articles though can be found that claims issues with any water, and some have much scientific data to substantiate the claims. But the sheer amount of misinformation is vast, and the best that is can be shifted for it all is as I tried to describe above - all water can be potentially hazardous, and the most common issues of why.
Speaking of RO though we have a AQUAtiva Spectra Twin with Chlorigon system in the basement, along with a additional AQUAtiva filter that uses 4.5" x 20" filters. Then under the kitchen sink there is a 6 stage reverse osmosis AQUAtiva PRISM system and water reserve tank. Anyway the point is we actually have the systems in our home but avoid using it now, for many reasons. The main one though is the filters are good for only 6 months (the RO membrane one and such though lasts 4 years).
We called to replace the sediment/charcoal filters and such awhile back for them and we was quoted about $300. I can safely say even with all the bottled water we buy for drinking, and the water gallon jugs I bought for my pets, even in 6 months it comes no where near $300 - so the system is worthless in that regard.
We did not buy the systems either, they came with the house from the previous owner which cost him $8,438.40 - personally I think he was a crazy to spend even a fraction of that amount.
Here are some photos of it all so you don't think I'm pulling your leg about it..