Fish dead

Question
I can't keep a fish alive... I'm not a novice, I just re-established a tank after moving a couple of years ago. The tank is 8 weeks old and has cycled well.  Amonia level "0" Nitrites "0" PH a little high 7.8 and falling. KH at last count 4 days ago 6 degrees. Temprature 78 degrees. By all accounts fish should be thriving. plants are doing well. the fish see to cimb and live at the top of the tank for a day or two before they float permanately. Any ideas???

Answer
My first guess would be ph shock.  How do you acclimate these fish?  I would have a bucket for fish.  Use airline tubing and tie a knot in one end.  Place the other end in tank.  Start a syphon where it drips 1 or 2 drops per second (adjust knot tightness).  Drip acclimate for 2 hours...then add about a cup of tank water every 5 minn to acclimate to temp...then place fish into tank.