Hi japanda,
'Nice of you to respond!
I ended up bringing the group indoors to a 70 gal tank and it's interesting to watch their behaviour. 7 babies came in with the goldies and koi; after a few weeks, 6 babies disappeared overnight, then the last one was gone during a workday. Those little ones were of considerable size so it had to be an act on koinabbalism (couldn't help myself, sorry:0).
Anyway, it's going well and I think this will become my new winter hobby.