guppies gasping at surface, not moving, ZERO ammonia/nitrite/nitrate!

[caykuu]

This is urgent; I woke up this morning, and three of my main females in my 10gal are just floating at the surface gasping.. they're not moving, and look near death! I checked ammonia, nitrite, nitrate were all 0, pH is 6.6, and I use carbon in the filter.

I didn't do any huge water changes recently. The only big "move" was I took out the amazon sword last week to put in my main tank.. does that matter?!
I also fed them brine shrimp flakes 2 days ago, which is new....

What's happening and need urgent help!

 

[caykuu]

.. Now I just moved my two important males to the 50gal, and I leave and came back, and the biggest one vanished and the other is hiding at the bottom not moving.

This is horrible.

.. I'm going to stop this guppy breeding thing for good now if my snakeskins all die again like the million other guppies I've tried to raise before them.
The pattern isn't even like they're new.. I have them for a month or two, and THEN they start to get sick and die for no reason. and it's not because of old age, because some of them are juvies.

 

[YaHemCheck]

Are you using tap water? Your city might but crap in there that kill your fish. ie chlorine and fluoride.

 

[caykuu]

Yeah my city has crap tap water, so I use RO water and add trace elements.

 

[caykuu]

Okay I just witnessed one of the females having multiple spazz attacks. Parasites?

Is Parasite Guard a reliable medication?

Still though, what's the cause of parasites and how can I prevent it for good?

I seperated the three females in a seperate container. Should I treat them all for parasites? Is it harmful to treat if they don't have parasites?

Would adding aquarium salt help? Can I add salt and medication at the same time?