Following, as I'm having similar issues. Started a tank up again about 2 months ago. 55 gallon, stocked it with 10 cardinal tetras, 10 fancy guppies, and a half dozen plants. I live in WA and with RIDICULOUSLY low hardness and sub-7 pH, unfortunately lost 7 of the 10 guppies. I've since gotten the tank to cycle completely.
Yesterday I added 10 additional, new guppies that all looked healthy at ACO. Current parameters after doing ~40% WC this evening, adding a little Seachem Equilibrium, few tsp of baking soda, and 5 squirts of Easy Green:
Parameters:
Temp: 76*
ph: 7.0
Nitrites: 0
Nitrates: 10-20
GH: 75-100
KH: 80-120
I noticed, about the time of completing my WC, two guppies very inactive at the top of the water, just under the surface, tail fins clamped but no labored breathing. I cranked up the airstone as much as I could to increase surface agitation and turned the light off, so I'm hoping these active, awesome little fish get better overnight and I don't have a repeat of last time despite fixing what I thought were all the issues.
Edit: Also, how do guppies do with phosphates? I've been having a pretty major Diatom bloom, so I bought a test kit and found 5-10ppm phosphates a few days ago. I scrubbed all the algae off the glass, and in my WC this evening got as much off the plants & substrate as I could.