Something is Wiping out my stock

[Aces11]

About 2 weeks ago I added three harlequin rasboras (from petco) to make my stock a total of 7 harlequins, 2 endlers, 1 female betta, and a bristlenose pleco. Since I added the new fish I've lost 2/3 new rasboras and 2/4 of my old ones and my pleco. Currently all 3 of my remaining rasboras have these white fungus looking spots on their fins and bodies. My betta also has a cottony mouth. I'm afraid I'm going to lose all of my fish except my endlers since they are perfectly fine as of now.
These are the best pics I could get:

 

[Fishdude3233]

What are the parameters?

 

[Aces11]

I dont have a test kit but everything was perfectly fine before I added the new harlequins and I performed a 50% wc after I began losing fish last weekend

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[Fishdude3233]

Tank size? WC schedule?

 

[Aces11]

Its a 20g high and I change about 50% every week or every other week



And I have 2 AC30s on the tank so it is a very low bio load

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[Aces11]

Well my betta died my remaining 3 rasboras still have fungus looking spots on them as well. Does anyone know what this is or how I can treat it?

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[anibis]

It could be colmnaris. Fungus-like tuffs on the mouth & before the dorsal fin (like the first pic) are common. It is also a rapid killer.

Best to treat it with a gram negative antibiotic like Furan-2 or Maracyn 2. This may affect your biological filter, so keep an eye on your parameters if you choose to treat.

I'm not an expert. No one else seemed to respond, so I figured I'd throw my opinion out there. Fish diseases can be a pain to diagnose.

 

[absolutangel04]

Well, since you have multiple spots with the fungus-y looking stuff, it is a bit hard to say. Fungus tends to infect the body of fish and can be treated with an antifungal treatment, while columnaris is a bacterial infection and needs bacterial treatment. I am posting a link for you to look at to see if it helps you determine what you are dealing with.
Aquarium Fish Fungus
You might have multiple issues going on at once as well.