Guppy,What is this

[fisher12]

Several of my guppies died over the last 3-4 weeks after getting new fish. When returning a dead one, there was a sign that said "these fish are being treated for fungus" on the guppy tank. Hate to treat the wrong thing because idk what this is,(and I have 2 gourami and Cory cat) but I figured it could the related. I thought it could of been dropsy first but then seen that sign, but I'm bad at this part. I thought they were all fine again, but couple days ago I notice the one in pic swimming in one spot not moving much. And the ones that have died I noticed they liked to swim(sit) next to the glass behind the filter.

 

[fisher12]

It's been 6 days since my weekly water change, he is trying to swim out of the aquarium at the top and then sinks back to the bottom. I've had. I don't expect him to be alive in the morning. And the few others that are left are beginning to sit at water surface, how all the others did a few days before they died. That is the first sign, then I notice swollen belly towards the front as you can see, they all seem to turn white ish and bloated