Yes I keep Koi, it dose`nt have to cost so much, if you do it DIY
Planned it, built it, on the cheap, lot of effort digging down for a 6ft bottom drain, gravity fed filter, Airlift pump return, Heated by ASHeatPump, My Koi are beautiful, pampered and contented.
In total I have about 50 fish in 2 ponds and 2 aquariums the garden and home has been 30 years in the making. Now in my retirement I can relax and enjoy my Koi.
My other interests are in ECO improvements, having installed solar hot water 23 years ago which has paid for itself 4 times over now.
My Hybrid heat pump system also heats the whole house and cut my gas consumption by 80% on previous years, hoping that will have a similar pay back on savings.