Why are my Molly's dying?

[Talleyrand]

I have a 25 gallon tank, fully cycled, about 3 months old. 10 days ago I added 6 Livebearers(swords, platy's,mollies mixed) then 2 more three days later to the existing fish listed below.
3 Rosy barbs
3 Bala's
3 Red tail sharks
3 Cory cat

(all fish are currently quite small, I know they will grow-planning on upgrading later as long as the current tanks go well)

Well in the course of the last 10 days 3 of the 4 mollies in the bunch of livebearers have croaked. To make matters worse, after the first two died, I read that the best cure for dying mollies was to add extra salt. I had some on hand so I added 4 tablespoons to the tank. To my chagrin the next morning I had 2 dead Cory cats and 1 dead Red tail. I had no idea salt kills cory's
The 3 swordtails, 2 remaining molly's, and single platy purchased at the same time seemed to be thriving until today when a previously healthy molly died.
Now I have:
3 Swordtails (1 Male, 2 F)
1 F Platy
1 M Molly
3 Rosy barbs
3 Bala's
2 Red tail sharks
1 Cory cat

Is the tank overloaded? All the fish are pretty small and I have a 170gph bio wheel power filter and a UGF.
Too may fish at once? I added the previous 12 fish all at once without a loss, after sacrificing some Danios to the cycling gods.

Nitrite-0
Ammonia-0
Nitrate-25 ppm
Ph-7

 

[Tempestuousfury]

I wouldn't say it's overloaded, though i believe the sharks you have in there get pretty big. Livebearers like salt, cories can't stand it. Mollies have special requirements. They are not very hardy. I believe they need a bit of salt and acidic water. I'm not sure though, so you should wait for someone else's opinion.

Also, barbs should be kept in shoals.

Do a water change and get the salt out or the cory might croak. Your choice unless you have another tank to place either the molly or cory in.

Add your fish to your sig.