Multiple/Mystery disease(s)? Black spot / scale rot / ich / parasites?

[TNC]

Greetings all!  I'm really hoping the collecting knowledge & experience here can figure this out.  I'm stumped

The basics:  these are "green lightning" platies.  I have four in a 10 gal hospital tank, separated when I realized they had a problem.  It started with a small black spot on top of one's head and I started researching it.  Soon after I noticed all four of these same type (no other platies in main tank) were looking ratty and I moved them out.  That was weeks ago and things have slowly gotten worse.  I have tried treating with Melafix first, then added Pimafix to a daily alternating schedule, no improvement.  I later noticed a few little while spots on one tail and then switched to ich-x for several days now.  They've all four just continued getting worse and now there are black spots on one or two of the others, one having very pale and scraggly looking scales with red showing through...

My first question is if they can even be saved.  Their activity and appetite are unchanged and totally normal still, which made getting any useful pics difficult.  I'm attaching the best I could manage.

I've regularly checked water params and all are normal, and did a 50% water change last night as part of the ich-x instructions.  Here's the current... very soft water params.  Seems I'd forgotten to check more than the API master kit does.  Will start trying to figure out how much SeaChem Equilibrium the tank needs.  I have a RO/DI system for tank water.
Tank size:  10 gal
Ammonia: 0
Nitrites: 0
Nitrates: 0
Hardness: 0-25ppm
Buffer: 40ppm
PH: 6.8

Now that I see the water is so soft, could that be hurting or preventing their recovery?

Any recommendations on how to adjust the hardness/buffer/ph without doing a chemistry experiment?  I tried the Equilibrium in the main tank once and the hardness went to what I wanted (I think) but the PH shot to 8.4+, which I did not expect...  I've let that come back down on it's own from water changes.

Hopefully I've included enough info.  Any and all help is appreciated.

[Colu]

Your kH and GH are  low for platys the lack of minerals in your water could be the cause I would add crushed coral to raise your KH looking at the pictures it could also be caused by a bacterial infection so I would also do a course of kanaplex in food

Edited November 11, 2021 by Colu

[quikv6]

My tap water is identical to yours. When I started with livebearers, they just didn't do well. I am not saying you should "doctor" your water, but I do, and there was an immediate improvement. I have been doing so for nearly 2 years now.

Take Colu's advice...it's spot on. Only thing I would add to it is some aquarium salt.

1) 1 TBSP per 5 Gal is a good start for platies.

2) Add crushed coral (preferable to the filter) to slowly raise KH. (PH will rise a bit too). You can also add to the gravel.

3) Equlibrium or Replenish will bring up the GH. Add it slowly, an don't overdo it.