Most fish are predators because there isn't much underwater vegetation to graze on...
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.
I have no idea about biomass ratios, but I'm pretty sure there are more species of fish that eat other fish than not.
Most phytoplankton, i.e. unicellular algae, is eaten by zooplankton, i.e. mostly by small crustaceans, like copepods.
The fish that eat other animals vastly outnumber the fish that eat algae or aquatic plants.
In open water environments, the kinds of bigger animals, i.e. fish, cephalopods, chaetognaths, cnidarians, ctenophores are almost all predators who, unless they eat each other, they are eating zooplankton that consists mostly of small crustaceans and of planktonic larvae of the animals that live on the bottom.
> The fish that eat other animals vastly outnumber the fish that eat algae or aquatic plants.
This is why I said "if you count all sea creatures". The original comment said "most fish are predators because there isn't much vegetation". As you say, there is quite a lot of vegetation, and there are many more sea creatures in that category than there are predators.
So, most fish are predators, but not at all for the reason in the original comment. It remains true that population and total biomass drop off exponentially as you go up in the food chain (which is really the only point I was trying to make).
The main reason while the majority of big aquatic animals are eating other animals, while the majority of the big terrestrial animals are eating plants, is the disparity in size between big animals and most aquatic primary producers, which requires at least one intermediate layer of small animals, who gather efficiently the small algae, packaging the nutrients and energy into bigger bodies, which can be gathered efficiently by the big animals.
The multicellular algae have bodies with a poor content of nutrients, because unlike terrestrial plants they do not need to store large reserves of nutrients in body parts like seeds or roots modified for storage, reserves that would be needed sometime in the future, after finding a place to germinate or after an unfavorable season passes. The stems of multicellular algae typically have only the purpose to anchor them to the bottom, to avoid being swept away by currents, so they do not need to contain much edible substance.
So only a few animals, e.g. sea urchins and some snails or chitons and few fish, have developed the special adaptations required to graze multicellular algae, like adequate teeth, because the reward for such an adaptation is not great and the places with abundant multicellular algae are typically restricted to regions close to shores.
Cows can get big by eating grass because most of the land is covered in plants, but tuna can't get big by eating seaweed because most of the sea is abyss.