It is ironic that after praying to Ganesha, considered the Remover of Obstacles we cause such terrible damage to water bodies by immersing the toxic painted idol and killing aquatic life! Please get an eco friendly Ganesha idol and save the environment.
All lakes and tanks in Bangalore have sewage water either pouring in directly or which is residual. Are you sure you want to immerse your beloved Ganesha in such muck? Complete the sanctity of your prayer by doing it in your own house in a bucket of clean water. Here is a link for contacts of Eco-friendly idols.
Have a great Ganesh Chaturthi but don't let your devotion kill a lake and the many lives it supports!