I know I am late to the party here, but, I keep Medaka’s in a barrel pond and they are as happy as Larry. The neocaradina love it to that they are spawning all over the place. The Medaka’s are 3rd generation i got from my mum’s pond in Herefordshire (she and her friends adore them) I had originally got her some chap from Tropco and she moved them from her tank to the pond…..my influence. They were a pale pinkish colour in the tank but now 2nd yr in the pond they are bright orange and breeding (strange as it is well below 25c at the moment) anyway they are loving the pond along with the newts and other visitors to her wild pond.
I took 11 fry out to grow on and join her smaller ones in tin tubs.
One of My Medaka’s has actually regressed back into being a natural brownish colour and is as active as the others. I am growing on 20 new fry in my heated aquarium. I swap the floating plants over every few days to get the eggs/fry