I would run a few feeder goldies in it over the winter to see if the
toxins are gone. It is very hard to get rid of detergent. A mild
bleach solution will neutralize it, but might harm your seals if left
on too long.
Rob G wrote:
Earlier this summer our well established pond was poisoned with
detergent - all very amusing for the perpetrator as the foam was
massive and we got complaints from the neighbours about it.

I've left it 4 months after a full clean out and refill, and am
wondering whether I could restock now or leave it till the spring ?

Rob