Answer: What is this called and why do they do that?

  When you see something out of the ordinary... 

Two fish swimming in close formation 

...the interested SearchResearcher should say "What??"  Or at least, "How's that?"  

As Isaac Asimov is alleged to have said, 

The most exciting phrase to hear in science, the one that heralds the most discoveries, is not “Eureka!” but 'That's funny...'

(I say "alleged" because as the Quote Investigator points out in his analysis, this particular phrase can't be found anywhere in his writings.)  

Regardless of its origin, the phrase rings true: seeing something and saying "that's funny" leads to interesting SRS questions.  That's the case here as well.  

In this case, there's a fishy peculiar behavior that I've seen multiple times.  Two fish of different species will often swim in perfect formation, like two fighter pilots flying in tight formation through the reef. Here's what it looks like in motion: 

I've seen this happen with many different species in different oceans and have always wondered: 

1. What is this kind of behavior called? 

2. Why do they do this? 

3. In the pic above, we have a hogfish being closely followed by a trumpetfish.  What other combinations of fish species might I find doing this?  

A query like this: 

[ two fish of different species swimming together ] 

leads us to quickly learn about schooling and shoaling.  A school of fish is a large number of fish of the same species swimming together in synchronized group.  

Here's an example of large school of Bigeye Trevally (Caranx sexfasciatus), which are frequently seen congregating in these synchronized, metallic-silver formations... 

By contrast, a shoal of fish is a large number of not-so-synchronized swimming fish (possibly of many species) some traveling one direction, some traveling another--like this group of somewhat disorganized fish:  

While that's interesting, it's not what we see in the video above--two fish traveling together in close synchronization.  

I modified my query to be: 

[ two fish of different species swimming together in close synchronization ] 

and learned that this could be called a "a heterospecific school or a mixed-species shoal."  That is, a fish group that has adopted synchronized schooling behavior. 

While schooling is typically defined by fish of the same species, size, and age moving together, mixed-species groupings occur when fish share similar body shapes, colors, or ecological needs, allowing them to gain the safety and energy-saving benefits of a larger school. 

That's interesting, but not quite what I was looking for.  I'm interested in pairs of fish (not schools or shoals) that are moving in harmony.  

When I give the query with the specific case of hogfish and trumpetfish, I get a very different answer: 

[ what do you call it when a spanish hogfish and trumpetfish swim together in sync ] 

The AI overview looks like this: 

This is the answer I was looking for.  A quick search on Scholar for literature about this behavior leads to many article (like this one: Predatory trumpetfish conceal themselves from their prey by swimming alongside other fish) 

So, shadowing or shadow hunting is the term for this kind of behavior between a largish herbivore and a sleek hunter.  

The researchers made a brilliant test rig to check out the idea that trumpetfish can escape detection by hiding behind a larger, bulkier vegetarian fish (like a parrotfish or hogfish).  Here's their video: 

We have our answer. 

BUT WAIT... there's more.  

Here's what I actually saw in the tropical waters on the Tubbataha Reef, the behavior that caused me to say "hey.. that's funny..."  

I saw these trevallys, also swimming in close synchronization with another all-black fish.  

This was striking: two fish swimming in what looks like shadowing, but they're the same size and shape.  Since they're so different in coloration, I just assumed they were different species that were shadowing.  (For example: see Giant Trevally spawing aggregation.) 

But they look identical, except for color.  What's going on? 

Or, as Asimov would say... "that's funny.."  

I grabbed a frame of this video and asked Google Lens to tell me what was going on.  Here's what it told me: 

A quick search to validate showed that this interpretation is correct.  Giant Trevally DO synchronized swimming when in a mating display.  I was lucky enough to see this behavior by diving on the reef within 2 days of the full moon.  

The black fish was a color dimorph (that is, a male that changed to black coloring for mating purposes), swimming close to a female for piscine seduction purposes.  

So... here's another example of very close in-sync swimming that's NOT for hunting purposes, but for mating!  

SearchResearch Lessons

1. Sometimes you need details to get to what you seek.  In this case, we needed the specific fish names to learn that these particular KIND of fish do shadow hunting.  Details matter.  

2. Sometimes what you see is not quite what you think.  Just because two fish are completely different color doesn't mean that they're different species.  

3. Sometimes what looks like the same behavior is NOT the same! Two fish swimming in perfect sync can mean very different things, depending on the species, the time, and the context.  

Keep Searching!