Guppy hanging out at the top of the tank. Help?

ok, so before anything, let me discuss my tank and its parameters.

Ive got a 10 gallon rimless aquarium in my apartment. It's planted with java moss, has a plant light thats set on a timer, planted substrate, and lots of seriyu stones for decoration.

I've done the aquarium thing quite a few times. My tank cycled for a month or so, I've got an HOB filter and I've been testing the water with an API liquid test kit daily. My stocking is simply 6 fancy guppies. four females and two males. I had them imported so they're not from a pet store, and they were rather expensive! 

Yesterday one of my males died. He was hanging out at the top of the water for awhile, and then it seemed like he just couldn't swim up to the top anymore and he just died. Keep in mind I've had these fish for a little under a week now. I thought it was just shipping stress and all the other fish looked fine so I got rid of him, did a 10% water change, and moved on with my day. 

this morning I noticed my other male doing the same thing. Gasping at the top of the tank and not really moving around. I checked my parameters immediately. 0 ammonia, 0 nitrite, 10-20 nitrate. I went to the pet store and got an airstone and added that. Any other advice? All the females seem perfectly fine. Should I contact the seller? Thanks!