A tale of two fishes

What’s the difference between the words “fish” and “fishes”,? This was explained to me a few weeks ago in a lecture about fishes (and if you feel I’ve left anything out, please let me know):
Fish (I’m just dealing with the noun form, pertaining to language used by aquarists, keep in mind) can mean an individual or a group of the same species of fish.
Fishes, on the other hand, deals specifically with the plural of two or more species.
So does this mean that the sentence “I eat a lot of fish.” is wrong if you enjoy eating salmon, tuna, yellowtail, barramundi, and other types of fishes? I don’t think so, but I’ll ask around.
Anyhow, on to the pictures. One fish pictured is a gar. The other fish is a goldfish. One of them ate the other one, and though the outcome was predictable, I was not expecting to see the following play out.
I don’t know enough about gars to hazard a guess at which species this is, but I can tell you that it is the King (or Queen) of the tank.

Maneuvering by using mostly its muscular pectoral fins, the gar slowly positions itself for a meal.

The attack is lightning fast, and devastating to the goldfish. Pressure sensitive canals are lined up along its snout, and function much like a lateral line, letting the gar sense changes in water pressure. It attacks by lining its prey up right next to its mouth, and then snapping it up with its needle-like teeth while flexing its body to one side.

For some reason, the gar spat out its meal, and started to drag the disemboweled goldfish around. It reminded me of a master walking his dog.
After a few minutes, the intestines finally snapped, and the gar finished its grisly meal.