Symptoms of lead poisoning in fish would be difficult to notice.
I found an article about a home in Saskatoon, Canada, with 23ppb lead in the tap water, after it sat in the pipes for 10 hours. I imagine that the concentration goes up when it remains in contact with the lead pipes longer. The city water is virtually lead-free before entering the home.
Aquarium water is usually a bit more acidic, and warmer, than tap water. Saskatoon's tap water is reported to have a PH of 8.6.
I'm not pretending to be a scientist, but I have found this topic on search engines with similar responses, that there is no observable toxic effects to fish. But if the above numbers are correct, that lead-free water gets to 23ppb after sitting in lead pipes for 10 hours, with a PH of 8.6, then I believe the lead plant weights could cause toxic levels of lead in aquarium water.
My particular weights had small amounts of rust on them, and I don't know for sure whether they actually contain much lead.
Have any of you, who use these weights, tested your tank water for lead?