Advice on Chemical Levels

Hopefully someone can shed some light on this ...

I have had a few dead fish which led me to check the chemical levels in my
pond (using the same kit I have for my tropical tank) and this is my area of
concern.

pH 8.8+ (that's as high as my scale goes)
Amonia 0ppm
Nitrite 0-0.25ppm

The pond is fairly new (about 6 weeks) with a small selection of plants
(both shallow and deep). At the deepest the pond is about 60cm and has a
30-40cm ledge al round. I would suggest that it's volume is about

400 US Gallons
350 Imperial Gallons
1500 Litres

We have about a dozen small Shabunkins (8-10cm).

I have a pump and gravity fed filtration system which handles about 2000
litres per hour. The pump is in the deep section with the return located as
far from it as possible to create as much circulation as I can.

Is the high pH something that I can expect from the pond/equipment not yet
being mature, the plants, fish or something that I have done?

Any experienced advice appreciated.

Thanks

Sean