Ive got some professional experience with lead ground contamination. Learnt more about the potential for lead poisoning on 1 project than i ever planned on.
Lead is very inert, especiallly when the surface layer has oxidised. Which is why is so useful. However lead is harmful to humans if ingested or dust/ fumes are inhaled. How harmful depends on how much lead. The levels of lead contamination on the project i mention would have required you to eat a bucket full of contaminated soil every day for 30 years before the lead would have built up in the body to toxic levels, but the site manager still had grounds maintenance wearing hazmat suits to cut the grass. Bit of an over reaction IMO
My wife is responsible for planning replacement lead pipework for plastic for our water company. In part because as it gets old and degenerates, lead can contaminate the water supply and cause health issues in people. But, those pipes have been there decades and not caused noticable health issues (so far).
Lead isnt going to disolve in water very much, but more will do so if your water is acidic. Its more of a threat to health when it starts to degrade and break up over time, and then bits of lead can find its way into the ground, waterways and atmosphere where it can be injested or inhaled.
Personally, if you are concerned, dont use lead weights. You need peace of mind. I would say plants wont be affected, fish arent likely to be, shrimp are more likely to be.
Ive just been reading about banana plants and everything ive read says tie or glue to a piece of rock or driftwood, and it will eventually root into the substrate.