It is lead, and don't worry about it so much. Lead is toxic but it's
not detrimental to the water or the fish unless they eat it, which is
unlikely. You can remove the weight and plant most varieties as you are
obviously aware. Several are difficult to take root though. Hornwort is
impossible, I hate the stuff. Java ferns eventually root if you tie the
plant to a rock or piece of driftwood using thread. The thread
eventually decomposes. So, maybe that's an option for you........

Oh, I sank some Ambulia using lead in a 10 gallon tank with 13 Cory
fry. I keep only 1/2" of gravel in that tank so I left the weights
on...... I've noticed no side-effects six months later.

Hope it helps.
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