Every metal will corrode. The metal will have oxides/rust on the surface witch can eventually leach into your water. The process will be speed up if you any type of electrical current in your tank, say a wavemaker. The metal itself is not toxic, it he oxides that can form on them. Someone correct me if im wrong, but I would stay away from any type of metal unless you were to seal it. You can find enamel coated fishing weights that make good weights. You will need to tie a little string around them and wrap them around the roots. Just an idea.