That's OK. GH at 50 ppm equates to 3 dGH, and KH (Alkalinity) at 40 ppm equates to 2 dKH. The hobby uses these two units, so it is always good to know both. This is very soft water. As the aquarium matures, the pH will most likely lower below 7, also good because soft water fish tend to prefer (= be healthier) in acidic water.
In post #1 you wrote:
These fish include ottos, pygmy corries, black neon and glass bloodfin tetras, celestial pearl and zebra danios, and endlers. [upside down catfish also mentioned.]
Taking into account what a couple of us posted earlier in this thread about compatibility...the Endlers being livebearers needing much harder water are out. The other fish are suited as far as parameters. Zebra Danios I would not consider, as they are very active swimmers, and some of the other fish are sedate and do not appreciate active fish around them. Seangee mentioned about celestial pearl danios, these are better on their own. This tank is presumably a 75 gallon. Pygmy cories are OK, but I find they tend to get lost in larger tanks, but you coould still have them in a good sized group, say 15-20. But there are over 160 medium-sized cories suited. Up to you.
Upside down are OK, in a group. Make sure you have plants or branches so this species can be "upside down" or it will be severely stressed.
Bloodfins I would not get, they have a tendency to fin nip and I can see the upside down cats being targeted.
Other fish to consider...most of the tetras (there are a few unsuited but generally), hatchetfish, pencilfish, some of the plecos (whiptail, bristlenose), rasboras, some of the gourami...lots to consider.