Only mollies struggling - why?

[Tankles]

I have a low-tech planted 26-gallon bowfront tank with about 8 platys (gold twin-barred, mickey mouse, sunrise variatus) and about 8 mollies (Mostly dalmatian). Additionally I've got about 2 or 3 oto's that have essentially been there since the beginning (set up April 2011) and 2 9-month-old snails. About 6 months ago, all of a sudden, the mollies began dying off one by one. A couple showed signs of dropsy, but the salt bath and maracyn 2 was too little too late. The others would just become lethargic and stop swimming. All were gone within about 2 weeks. But every other fish was unaffected, eating and swimming just fine.

So I held off on mollies for a bit. about 4 months ago I added a dwarf gourami. No issues. Recently (6 weeks ago) I added 6 guppies (2 male, 4 female), and 3 mollies (male black, female dalmatian, female silver). All were doing just fine. The black molly hid for a few days, but the other two were coming for food and saying hi and acting like fish. Soon the black molly would come out for food too, just not coming to the front of the glass, but otherwise fine.

Now in the last week, after 4 weeks of being just fine and normal, all 3 mollies died off in quick succession. One each day for 3 days. But every other fish (8 platy, 6 guppies, 1 gourami, 3 oto's, 2 snails) are all hunky-dory no sickness no issues.

What would cause only mollies to have issues and so quickly?

Water is .25 ammonia on that day (probably due to dead fish), since today it is 0, 0 nitrites, 40 nitrates (bit high, but shouldn't only affect mollies, right?), temp 76. PH is lower, around 6.5. I run dual filters of a AquaClear 70 with extra bio-media and an Aqueon QuietFlow 30 with a live pothos growing out of it as an extra natural filter

Thoughts?

 

[Fishfur]

pH might be the problem. Mollies are from brackish water originally, which means they prefer hard, alkaline water conditions. They are generally very adaptable, but if there was some sort of infection, perhaps, to start with, the acidic water might well have made them extra susceptible. And whatever it might have been, from the sounds of things, it is still there.

Perhaps mollies are not a good fish for your water conditions ? You can try raising the pH, but if your tap water is like this naturally, it's quite hard to maintain the change and having it bounce around is very stressful for all the fish. Stable conditions are always preferable.