Dying Fish - Quality Images - Help Me Identify The Problem

[gabriel.mihu]

20g tank has been established for a few good months now, but has gone through a fish-in mini cycle (about a week) when I changed part of the substrate, about 5 weeks ago.
History:
4 weeks ago:
- removed 5 buenos aires tetras (been there for 3 months)
- removed 4 giant danios (been there for 3 months)
- introduced 2 swordtail females and 4 neon tetras
-> in the tank: 2 swords, 2 cory cats, 4 neons (1 died in 24-48 hours), 2 fancy guppies males (died in 1-3 days)
3 weeks ago:
- removed part of the gravel, changed with black, coarse sand
10 days ago:
- water parameters back to normal (7.6ph, 0 amonia, 0 nitrites, about 10 nitrates)
- introduced 2 fancy guppies males
- 1 sword dies
2 days ago:
- added 3 mollies, 1 sword male
 
Right now, the tank has: 2 cory cats, 1 sword male, 2 fancy guppies male, 2 sunset mollies, 1 lyretail molly, 3 neons; water params ok; 20g tank, lightly planted; internal filter
 
PROBLEM:
 
Yesterday, everything was fine, even last night. I reduced the filter flow, as the power head generated a too strong current, and the fish seemed kind of tired. They seemed to love that, and mollies/guppies started swimming around.
This morning I see my sword female is dead, and all I can see is the whitish spots on her. Check the images below and let me know if you have any advice, if this is something common, if this is treatable or not, or if it doesn't mean anything.
 
Any help would be appreciated, as I don't want to lose my fish - they are pretty nice, colorful, and they seem in good health at this point. I love the lyretail, and really loved the swordfish that died, too - she was a really fine specimen, as is the lyretail.
 
I want to introduce 5 more neons, but they're in my quarantine tank for a few days, just to be safe that they don't have any ich before putting them in the tank - I just got them a couple of days ago from the fish store.
 
I don't see any sickness signs on any other fish, they seem to be doing fine: swimming around, not too agitated, and eating just fine the flakes. Only on the guppy in the photo I see the pelvic fin a bit white - I don't know if that's a sign of anything.
 
Any advice is welcome.
 

 

 

 

 

 

 

[bluesword23516]

The male guppies fin seems fine.  I believe it is just part of his coloration.  Im not sure about the female that died though, sorry. 
PS you also have a male swordtail, if you didn't know that. If you did, then ok

 

[gabriel.mihu]

Thanks for the reply. It's hard to identify fish illnesses when you're not an expert, and google only helps up to some point. All fish seem to be doing fine at this point. Hopefully I won't lose anymore.

 

[ReiRei]

I'd love to help, but this isn't something I've experienced with my own fish, so not sure my advice would be any good.
I do hope someone comes along and does have some though
 
Just wonder what camera/lens that is? Very clear photos.

 

[gabriel.mihu]

I'd love to help, but this isn't something I've experienced with my own fish, so not sure my advice would be any good.
I do hope someone comes along and does have some though
 
Just wonder what camera/lens that is? Very clear photos.

 
That's ok, Thanks for replying, though.
 
As for the camera, the camera is fancy, but not the lens. I could do the same thing with a point and shoot... depends on the technique you want to use. In this particular case I used a DSLR with macro extension rings and the lens. Nothing too fancy or too expensive (by my standards).

 

[attibones]

Looks like an issue with the slime coat, possibly a fungus. Please specify your water parameters, including temperature. Have you added any medicine?
 
Your tank is pretty full, you know. Mollies and swords really need more space when with their own species, so you've got them quite cramped. Plus two schooling species (I know you stated you would like to fill out the tetra school), but you really don't have the space.
 
In the meantime, do a nice sized water change, make sure your tank water is at 0, 0, 0-40, mid to upper seventies (can't recall specific temps for these guys), and, of course, keep a close eye on your other fish.