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.