You keep saying that the water test results are perfect... But you never state what they actually are. Although, tap water, depending on your location, can test 0-0-0 on the ammonia, nitrite, and nitrate.
As long as you never add an ammonia source, it will stay that way. Fish, however, are an ammonia source. So is the food they eat, or more accurately, the food they miss that falls to the bottom.
Did you read the article on cycling? What kind of cycle did you do?
And no, turning everything on for three days does not cycle a tank. It moves water around, and that's it. Cycling, as pointed out in the fine article by R.Bishop that I linked to involves the buildup of nitrifying bacteria to deal with ammonia. It does not happen in three days, and it does not happen without a source of ammonia.
I would strongly suggest that you read the article linked, and/or do a forum search about cycling. If not, you will be throwing money away and killing fish in a toxic cesspool as the ammonia begins to spike.
Also, regarding the buffer you added.... why? Because some test strip said you needed it? Did it first ask what kind of fish you were keeping? I bet the test strip also said a pH of 7.0 is ideal. Some of the more delicate species that have special needs would die in that. Quickly. There is no one answer for ideal conditions... the best we can hope for is to keep things steady and let the fish adapt. This means working with what we have, not adding chemicals.
If you try to alter the water chemistry by using additives, your tank will become a glass box into which you pour money until either you run out of money, or all the fish die off.
By the way, you asked earlier about the salt... It's as much of a waste of money as the snake-oil chemicals are unless you are using it to treat for a specific disease. And just as bad for your fish in many cases.
