-It's plenty big enough to keep koi if you dont over stock. I cant work gallons but that's nearly 20m3. If you give a koi 2m3 he should be happy and grow well, dont ever give them less than 1m2 per fish or you'll soon have health problems.
-Yes, yes, yes, you must have a properly organised filter system, vortex, UV, mechanical and Bio filters, (there are lots of different ones/price ranges on the market)Koi produce a lot of (bleep), to put it simply, this must be removed constantly for them to survive. Get informed about filters first and plan and build your pond carefully before buying fish.20m3 of water needs a lot of filter. Buy a Koi magazine, I find 'Koi Carp' very useful, talk to your local koi dealer/specialist or other people who keep koi.
-Rain water alone is not such good idea, it has no Calcium which buffers the water, (even if it's not acid rain!) and no other minerals. Koi need a ph of between 7.5 and 8.5 which is slightly alkaline, (rain is about ph7) and the ph must be stable, calcium helps this. Better use purified (there are special purifiers for koi ponds) tap water. We are lucky here, we have untreated mountain water in our taps.
-Koi are better off without catfish, or put another way they will take up the room of a koi, (dont know what conditions they need, but they will also pollute the water.)
-Start with 2 or 3 fish, dont add a lot at once as the filters need to build up slowly, add a filter starter to help.
I could go on all night, (I've been keeping koi for nearly 20 years and I learnt the hard way, we are still improving on our filter system!)Wish you the best of luck.
regards
maryclare

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