Fish in Hiding

I have a very small pond, about 120gal and small water fall. We live in N.
Illinois. 4 Koi, 3 Schubunkins in pond. They always swim aound, even in
winter, and beg for food. I use a floating water heater and all have now
survived three winters OK. The heater is now out of the pond. Ten days ago
we had a cold snap, down to 25oF. The pond did not freeze, but the fish
went into hiding and have not come out from under a stone shelf. This is
abnormal behaviour. I changed 60% of the water adding chlorine and
chloramine remover. Still no movement. Is this normal? What should I do?

Thanks,
Martin