15 ltr square RO tank

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.