New Tank - Sick Guppies - Water Quality Issues

Question
My husband and I just started up our very first aquarium. It's a 20 gallon tank. We have a bubble maker, a thermostate/heater, and new filtration system. For decor, we've got colored gravel, fancy stones and bigger natural-toned rocks, no wood, plastic plants, and an bubble airrator.

We choose 7 fancy guppies to start our tropical freshwater tank (2 females and 5 males) last Saturday. We treated our tank with 1 tsp of salt/gallon right before we put the fish in. We didn't do any other water treatments (we have well water). Our water was at 84 degrees at first by accident, but we gradually lowered it and the water temperature is now stabalized at 77-78 degrees.

At first all was well, then I noticed that one of our males wouldn't eat. He was also trailing the other guppies around close to their tail, keeping up with them and if it was able to get away he'd find the next guppy and trail them. This all started the Tuesday (today is Friday).

On the Wednesady day I bought water testing kits: ammonia and PH. Ammonia was "ideal" and PH was registering at least 7.6, the test kit doesn't register higher than that. That same day I used a chemical (2 tabs for the 20 gallons) to bring the PH down to 7.0, but it was back up again yesterday. Yesterday the ammonia was "stressful" so I did a 25% water change (kept the tempurature at 77 degrees, but didn't do any water treatments). PH is still at least 7.6, but the water change brought the ammonia down to less than stressful, but not quite ideal.

I stopped doing water changes though, because now all of my guppies are floating at the top of the water! The one male still isn't eating and he's developed a wobble that starts at his head (I know their tails always wobble, but it looks like he's shimmying more than the rest). He's also stopped trailing the other fish because they're all just floating at the top.

Please say you have some advice, I really don't want to kill these guys!

Answer
Hello,
Ammonia is the number one killer of fish.  It is a waste of money to try and cure them if your ammonia level is up.  Check your nitrites also.  A cycled tank must have zero ammonia, zero nitrites, and nitrates should be low.  You will have to continue doing water changes until all three of these are safe.
When cycling a new tank, you must let it run for at least 3 weeks with no fish in the tank.  Buy a used sponge from your fish store, do not rinse it, and put it into your filter.  A used sponge is full of good bacteria that the fish need to survive.  After, when you clean your filter, you always rinse your sponge in the water you have removed from the tank, and never under hot water or tap water as this would kill the good bacteria.  You could also ask your pet store manager to give you a handful of gravel to help you establish your tank.
Adding too many fish at once will change your water chemistry, and the result is death to all fish.
Once your water chemistry is right, you add one fish, wait one week, remove 25% of your water, add new water, and if your water chemistry is safe, add 2 other fish, wait one week, repeat, and so on.
Never use chemicals to reduce your PH.  They do not work, and cause harm to fish.  A fluctuating PH is very bad for fish.  Guppies can handle 7.6PH.  If you really want to bring it down, add driftwood to your tank.  Rinse the driftwood well, and leave it in a pail of water for a week or so, until the brown tint is out of the water.
For now, the only thing you can do is buy a used sponge, ask for gravel from an established tank, do water changes of 15% everyday, and maybe you will save your fish.
You cannot medicate while your water chemistry is not right.  
I hope that some of the little guys will pull through.
Lynda