Small 20 litres fish tank aquarium complete tropical setup

So the pond is filling up at the moment, and I’m going to leave it like this for two weeks or so. So I need a PH kit is there anything else I need before putting koi or gold fish in the pond

Want this but need to get the tank dialed in first

I have about 6 of them now need a couple of new tanks

Anybody know of a big tank for sale?bigger the better asking for a friend.