our first fish loss

Well, this morning we woke up with a floating shubunkin... He had been
acting odd the past couple of days, 2 days ago he spend most of the day
just floating up in the shallow water on our gravel shelf, maybe in 2"
of water. You could reach in and touch him and he would just hang
there. Yesterday we barely saw him, actually though he got eaten by
something as he didn't make an appearance for 8 hours.

This morning he was floating and gone. Of course this is the fish my 5
year old picked out, so there were many tears this morning... The comet
and other shubunkin seem to be doing fine, they are active and eating.
Just a sick fish perhaps?

Water tests seem not too bad, ph is high, in the 8-8.4 range. Should I
try to lower it? Pond is 120 gallons and only 1 week old. I'm wondering
if some stones we put in or the bricks holding up planters may be
leeching something keeping the Ph high? Nitrite levels are good,
nitrate levels are safe and total alkalinity is good. The only other
thing is that the water tests as very hard. These are test strips of
course, tonight we go to the aquarium store and have them run more
accurate tests on the water.

So, the other fish seem fine... Should I worry about the Ph or water
hardness or just let nature take it's course?