Dechlorinate immediately, add the bacteria the night before Koi arrival. Get several floating plants (water lettuce, water Hyacinth) not sure if you call it the same over there. That will suffice for sure. I clean out my pond once a year, but gets a net over it for fall and winter. Pond is drained, Koi in a large tub with filter. Powerwashed, rinsed and refilled with tap water. Dechlorinated, add bacteria, add plants, add fish. Simple as that really. Koi and alot of goldfish are almost "bullet proof". I need to get some pics of my Koi up...can't find the camera cable
125g semi planted. 2 silver angels, 14 silver tips, 16 white fin ornatus tetras, pair of Nicaraguense, pair of nototus severums, trio of firemouth cichlids. 2 FX5s, 1 1500gph circ pump, 2 48" LED lighs, and a HUGE piece of driftwood.
650g fish room. 15 30g tanks, 150g sump.