Tropical aquarium newbie HELP!

Hey,

I doubt there's many tropical aquarium experts on here but thought I'd give it a go...

I've recently purchased a 7litre tank from pets at home. It came with a heater, filter and light.

I filled the tank with tap water and put this 'starter aquarium' stuff in it (stress release for fish, makes tap water safe, adds good bacteria etc). I've also purchased a live plant on wood and a moss ball.

I left it to settle for a few days, and then I purchased three tiny red cherry shrimp and three guppies (one guy, two girls).

I added the fish yesterday. They seemed happy at first, but then I noticed the female fish started to become really shy (hiding and laying here still at the bottom of the tank)... a couple of hours later, all three fish were at the surface of the tank as if they were trying to breathe. At first I thought it might be because they were hungry, so I fed them, but they continued to stay at the top of the tank, floating still.
I left it over night, and then this morning I woke up and found the two female fish were dead the male fish is alive, but he's not very lively like he was yesterday. I've done a 50% water change, and added some more good bacteria solution stuff. (Only a few drops), he perked up for about half an hour and now he's back at the surface and looks like he should gasping for air.

I've ordered an air pump with air stone which will come on Friday, as I can only assume there is not enough oxygen in the tank. The shrimps seem fine.

I've checked the PH level it was about 7.8 which I believe is on the higher end of the 'safe zone', but again, this solution I've put in is meant to help with that. The temp is in the safe zone.

Any suggestions!?