I walked by a stream today and saw some watercress floating around. I picked some. It looks very healthy with roots and all. I am thinking about putting them into a goldfish tank. Will the harm my fish?

Shouldn't if you clean them, and the goldfish will enjoy the nice salad offered to them.

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220 Tank build
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You were exactly right.
I forgot to feed the fish this morning and when I came home in the evening, the watercress was mostly gone.
The watercress I picked was like 18" long with roots and leaves.
Now only a few inches of the root is left behind.
I initially thought the watercress would make a good decoration.
But it ended up being a salad as you have predicted.

Goldfish are herbivores so its to be expected. They love duckweed also. You might try anubia and java ferns, but not sure if they will last.

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SUCCESS IS MEASURED BY YEARS, NOT MONTHS!!!
220 Tank build
http://www.aquariumforum.com/f15/sta...late-3253.html

Well at least your fish got a healthy free meal

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When in doubt, do a water change.

I have some in a 25 gallon Aquarium with 3 small comet goldfish, I have a large amount at the top of the tank on a wire shelf with its roots in the water and its top out. I also have 3 medium sized one completely submerged so the fish can get at the leaves, I am hoping that by the time the submerged ones die I will have grown an equal or close to equal amount to replace it with. The goldfish seem to like hiding and swimming around in the tangle of roots.