I have a small pond about 80-90 gallons, with 6 goldfish and a variety of plants. I was thinking about adding some crayfish. Would it be to small for about two or three crayfish or is it just a bad idea all around?
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The crayfish may attack the goldfish. Friends and I have found out the hard way they like to claw at the tails and fins.

and the question you have to ask yourself is "will I beable to see them laying on the bottom ". And if one dies its one more thing you can't see that will foul the water
my$o.o2

Crayfish love to grab at things with their claws so they could easily harm the goldfish especially in such a small pond. The crayfish may not get enough food in such a small pond. They are also prone to jumping so they may end up bouncing around on land next to the pond. Also, raccoons love to eat them and find them easier to catch than fish so if you have raccoons, the crayfish won't last long. As Wayne said, you'd probably never see them once your pond was greened up (with algae and plants) for summer. If you live in a cold area, you'd have to bring in all the animals for winter in a 90 gallon pond. I've never put crayfish in my ponds but did put ghost shrimp in my 1800 gallon when it was new. I saw them a few times that summer but never again. Crayfish are a bit hardier than ghost shrimp. It's your pond and your decisions but I would say not to add the crayfish. To enjoy them, try a species-only aquarium. Good luck!