Question

An aquatic plant introduced from america to check pollution turned out to be troublesome weed jn indian water bodies

Answer 1

That plant is Water hyacinth (also known as Terror of Bengal) which was introduced in India because of its beautiful flowers and shape of leaves.
It grows in stagnant water and requires large amount of dissolved oxygen due to which aquatic animals like fishes have to suffer and resulted in declining the no. of fishes.

Due to lack of its natural competitor it increases it's no. into hectares of land