Question
i have drained my pond but have a small pool of water left.can walnuts be used to kill remaining fish and will it remain in water for a time.

Answer
Yes walnut hulls were used by indians to kill fish long ago . They contain a chemical called jugalone.

Jugalone Toxin

A handful of green hulls thrown into a pond or a lake has been known to kill the majority of the fish inhabitants. Crush the husks of "green" black walnuts and sprinkle them into sluggish water or ponds for use as fish poison. The toxins are so powerful that tomatoes, apples, and some other species cannot grow near large black walnut trees.

Poisoning was generally done in stagnant pools or slow-flowing streams and rivers, but has also been used by Californian Indians in saltwater environments for octopus and low-tide shellfish fishing (Heizer, 1953), as well as for catching fish trapped in inter-tidal pools (Bearez, 1998). Poisoning was such an effective method of harvesting fish that it was not uncommon for groups in some areas to purposefully dam small streams and ponds for this express purpose. Reducing water flow ensured that small quantities of the poison worked at maximum efficiency by minimizing dilution. In cases where the poison was not entirely effective, such as a restricted supply of poison or where stream flow diluted the strength of the poison, fishermen with scooping baskets, spears, or bow and arrows aided in capturing fish that were not fully drugged.