Help! What Am I Treating?!

Hello all.  My name is Anna.  I'm relatively new to fishkeeping and brand new to the site.  Was really hoping it would be a while before I needed some expert advice but unfortunately I need some help.  My fish are getting sick and I am not sure with what.

I have a 36 gallon bowfront tank. 
temp 82
pH 7.6
Ammonia 0
Nitrites 0
Nitrates @ 20

I have 6 rummy nose tetras, 3 kuhli loaches, 1 pleco (bristlenose I think), and a couple ghost shrimp

Everyone made it through cycling with no problems and the above are my current numbers.

I used to have 4 bettas in this tank also (1 adult female, 2 juvi females and 1 juvi male).  All seemed to be getting along with no problems. Last Sunday morning, I found 2 bettas dead (the adult female and the male).  By the evening, another female was dead (still no sign of any disease).  thinking that the remaining female may have decided that she hated everyone, I removed her from this tank since she was chasing the tetras a little and put her into another tank.  The little tank's numbers were pretty good too.  temp a little higher (@84); ammonia 0, nitrites 0; nitrates higher at 80.  By the next morning, she was floating nose down, listless with a greyish "patch" over her abdomen.  The patch didn't look fluffy.  It covered at least half of her body.  In a desperate attempt to save her, and the 5 second search online I was able to get in before leaving for work, I put medicine in the tank (API fungus cure).  Sadly, it was too late for her and she died during the day.

I had a similar problem about a month ago when all of my neon tetras were decimated by something similar.

This morning, looking into the 36 gallon tank, I have noticed that one of the rummy nose tetras is exhibiting the same symptoms.  listless.  hovering in one spot. and has a greyish white patch of discoloration on his tail.  almost looks erosive but I cant be sure. 

Anyone have any ideas as to what this is? I have fungus cure.  I have triple sulfa.  I have a parasite cure.  I have melafix.  what the heck am I treating? This seems to move quick as it killed off all my bettas in 24 hours. I really don't want to lose the rummies.  the pleco has seemed fine through all of this and he has been in the tank with the neons, bettas and now the rummies through all of this.

Any help would be greatly appreciated.  Will attempt to post a picture is a sec.

It is quite possible that the first two bettas killed each other. Despite what some shops say, males and females should never be kept in the same tank except for very briefly when breeding and even then only under constant supervision.
Female bettas should either be kept alone or in groups of 6+. One female will be dominant and with 6+ the aggression will be spread among several fish not just one or two fish being picked on constantly. And there's also the problem that short finned males are often sold as females, making the situation even worse.

But having said all that, the grey patches sound more like columnaris, a bacterial infection despite often being called fungus. This can kill fish quickly. Melafix is no good for things like this, it is more useful for stopping infections getting into a wound. And bettas don't do well with those medications ending in -fix, their labyrinth organs are affected.
You need an anti-bacterial medication. But I don't know anything about triple sulfa though API's website says it is for bacterial infections. If you have some, I'd try that.

Neons don't do well in new tanks. This is quite probably due to the conditions at the fish farms as a lot of them constantly dose antibiotics then when the fish leave the farms, the antibiotics stop and they succumb to any disease going.

So sorry to read about your fish.  I hope they make it over your trip