Alkalotic PH = sick fish?

Hello, I'm the new happy owner of a 29 us/gallon fish aquarium. I brought home my new tank and set it up according to the provided manuals (I bought a kit). 1 week later I went and purchased 4 black neon tetras, 4 cherry barbs, and 1 rainbow shark (thought it looked cool). The main reason I got these fish is because they were on sale and I wanted to make sure I could keep them alive. Unfortunately I couldn't. I have 2 cherry barbs left, 1 black tetra left, and my rainbow shark is gone. This all happened within like 3 days. Every morning I notice a fish or two stuck to the filter screen. I then noticed that the other fish were not as active and were hanging around the air hose. I checked the chemicals in my water and noted that I had a PH of 8.0 and a second test reading of 7.8. My nitrate, nitrite, and ammonia levels were all normal. The temperature is around 78 degrees F. I need help, everything I have read states that the PH is a little alkalotic, but not high enough to be killing my fish off so rapidly. What can I do??