Interesting question as I think I have got my old man to do the same to their pond, about 5*8m spring/run off fed. The major problem there is nitrates and phosphates
Plants:
Lemma spp & Azolla filiculoides for the surface to keep the temps lower and provide shade - these will need to be kept under control or may cover the whole pond
Ranunculus amphitrichus for some of the side, like the water
Some of the sedges might do ok, "cutty-grass" carex geminata?, carex secta?, carex flaviformis?, carex coriacea?
Scented broom if there is room?
Parataniwha if there is shade
Sweet-scented grass
Flax
Fish:
Inanga - wont breed but don't mind slightly warmer water and easy to get hold of
Banded kokopu - wont want it too warm, and like shelter - I believe can landlock
Mudfish?
Common bullies - might breed
Crain's bully - might/should breed
I wouldn't catch non-diadromous Galaxiid as their populations are much more at risk than the 'white-bait'