Did you use the medication in the same tank ? The different types of medication ?
It is very difficult to say what medications does to the water, and what different types of medication do, – they may mix into something really bad.
Thats why I think the best cure is get them out of that water, and into a new tank.
The reading seems fine, but they dont say anything about these kind of things, or amonium for example, which sometimes also can happen in a low ph, acidic tank, – that for some reason you have a peak in amonium, which will poison the fishes.
A number of things can happen like that – which you cant really measure, or which at least I think your measures are not saying anything about