What is the most common aquatic plant?

Algae are the oldest and most common type of aquatic plant. They are found in the ocean and they have no roots, stems or leaves. Algae are extremely small but they are the basis of the ocean food chain.

Food chain, you mean all fishes eat algae?

Yes it is a food chain but not all fishes eat algae. These are some of the fishes that are algae eaters:
Bristlenose Pleco.
Siamese Algae Eater.
Chinese Algae Eater.
Otocinclus Catfish.
Malaysian Trumpet Snail.
Nerite Snails.
Mystery Snail.
Amano Shrimp.