Ponds can not always go to plan as buying fish does come with certain pitfalls.
If you introduce new fish to an established pond it is not uncommon for a few to die as they are not used to the different water quality... (imagine living on planet Earth for several years then going to Mars..air quality is totally different )..same applies for fish.
They are kept in a very unreal enviroment probably a plastic barrel in a garden center..then transfered into a totally different world .They also have diseases and can pass them on to other health fish which then die.
Always start off with cheap goldfish as they are normally very hardy/pretty cheap and give years of pleasure as they can live in excess of 10 years and get quite large.
If all survive 1 year to next you have established a health pond ..trick then is keeping it that way.
Our pond is 18 years old and I have only ever put gold fish in from new.Each year they multiply and recon we now have 200-300 from a family of just 6.Some are 6-8 inches and all healthy..yes some years normally after winter we find a couple dead on the top but that is life and nature.Pond is 8 mtrs x 5mtrs and 1 mtr deep and like yours is raised as our ground is too hard to dig any depth into.Have 2 filters with uv running and a small fountain which is just to make a noise in the garden and keep some surface air.No water lillies as they dont like moving water but we have loads of pond weed for oxygen/2 very large clumps of yellow flag iris and 1 corner is full of yellow marsh marigolds in the spring.
I don,t ever feed the fish as they have more than enough natural food with flies and bugs and last year we even found some newts which are protected species so they now need to stay.
Enjoy your new hobby.