Although people would like more depth, you can in fact keep koi in
your pond. The rule of thumb is 1,000 Gal for the first koi and 100
for each additional one. Your pond is something like 7,000 US
gallons. It is a bit on the shallow side, but if it will not freeze,
they will survive fine in it.

I am trying to imagine a metal grid for an 8 meter pond!

A lot of us would want to encourage you to have your gardenr work out
some sort of veggie filter rather than only a mechanical one. Saves
work!

You can fit a bottom drain and have a waterfall, even if it is not
built in.

Keep us posted, or post some pics or drawings somewhere. Folks will
have lots of ideas for you.

Our pond is 12 x 22 x 2 and has handled the koi fine for more than a
decade. You can see it on our google profile link.

Best to you.

Jim