Ok so I just looked at your post.
Your temps and RH is fine man. That should not and will not cause problems around those temperatures.
Those strips. Well yes, that could very well be the culprit. When you feed with a 5.4 one feeding, then a 6.2 the next feeding. Then back down to a low 5.1 (for instance) PH fluctuations, can cause major problems. Now that your feeding daily, having that many up and downs, could cause the problem. The strips are hard to read sometimes. I’m sure you have but it’s worth spending some money on a good ph tester.
Secondly, I think you should maybe skip a day of feeding with the troubled one. Wait till she starts looking depressed and droops. Then hit her with 2 gallons of fresh water (un PH’d water) following 1 gallon with water PH’d to 5.8
Let that sit for a day or two again. Then continue feeding regime from there on out.