There is a lot of phytoplankton, plant-like algae. Though apparently most fish don't eat it directly. Zooplankton (which consumes phytoplankton) seems to be a more common food source for fish. You could call a fish eating zooplankton a "predator" because zooplankton are non-plant microorganisms, but that stretches the definition.

Anyway, tuna don't just eat zooplankton, they eat smaller fish. And most fish are definitely not predators of this kind, because that would be mathematically impossible.

If you are counting individual fish (independent of size) and you count all sea creatures, I think this is false. The "vegetation" are small creatures like phytoplankton, and the fish that eat them probably vastly outnumber the fish that eat fish. Each stage of the food change contains about an order of magnitude less fish than the level below.