Clark 6.86 equals GH of 5.5 dH. This is one unit the hobby uses, the other is ppm which would be 98 ppm. This is certainly soft water. As I expected. We don't have the KH, but it tends to match the GH (there are always exceptions, but this is normal) so that means soft water with an expected low buffering pH capacity. Therefore, what I explained in my previous post is absolutely normal and to be expected.
Now, guppies are livebearers, and livebearers generally do better in water that is harder than this. However, guppies are not as delicate as some would be and might be OK here. I'll tag one of our members
@emeraldking who frankly I consider an authority when it comes to livebearers, he can provide his insight.
Another option would be to consider the hundreds of soft water species that would absolutely thrive in this water. And I do mean thrive. Being a 100 liter tank though does have space limitations. But there are many "nano" soft water fish species.
BTW, there is a "Calculator" link on the right side of the screen that allows you to calculate several parameters and tank sizes. Very helpful.