This article is part of our Pets special section on scientists’ growing interest in our animal companions.

Many students begin veterinary school with career aspirations that date back to childhood, when they fell in love with the idea of ministering to cats and dogs, or horses, or the exotic animals at the zoo. Jessie Sanders arrived at veterinary school with a more particular passion. “I was the one weird fish kid,” she said.

It was an interest that had surprised even her. In college, Dr. Sanders had started volunteering at an aquarium, hoping to work with the whales. Instead, she found herself assigned to the fish team — and falling hard for her finned charges.

“I just love fish,” she said. “I love the way they’re built. I love the way they interact with the environment. And there’s still so much that we just don’t know about all the little internal workings.”

Today, Dr. Sanders runs Aquatic Veterinary Services, with patients that include carnival goldfish, pet store bettas and prizewinning koi worth tens of thousands of dollars. Last year, she became one of the first 10 veterinarians to receive a board certification in fish practice, a wholly new accreditation.

Dr. Sanders spoke with The New York Times about life as a fish veterinarian. Her story was based on two conversations, and her responses were edited and condensed.

I’ve done nothing but pet fish for 10 years, and it’s been awesome and challenging. I like the challenge of setting everything in an underwater environment. And the amount of personalities that you get in fish — they have so many little quirks. Some of them are super chill and nice, and some of them are complete terrors.

We had a hospital for about three years. Unfortunately, a 24-hour gym moved in and shared the adjacent wall, and they liked to play their music all night long. Fish have an organ known as a lateral line that picks up on vibrations; it’s how they’re able to sense predators, swim in a school together. Obviously, having rock music played at you all hours in the night is very stressful. Anything connected to that wall we lost within the first month they opened.

We just have a mobile practice now. We serve the greater San Francisco Bay Area. I’ll drive from three to eight hours in a day. When I get there, it’s the same as you taking your cat or your dog to the vet. We’ll have a discussion: What’s been going on? Have they been eating? Is there anything in particular that you want me to take a really close look at?

The most common “disease” that we see in fish is actually poor water chemistry. Like the air that we breathe, the water that a fish swims in is critical to their overall health. If you’re breathing in nothing but pollution, you’re going to be prone to more illnesses. So we check the water chemistry; if it’s terrible, the fish are already stressed out. I don’t want to put hands on them because that could make things worse.

Then you’ve got to catch the fish. I have a bunch of different nets. The cute little square aquarium nets for tank fish — I usually use one on each side of the fish and kind of squish ’em together. In bigger ponds, I use seine nets. They have floats on the top and weights on the bottom. I have ponds that are so big that I have to use two nets and get in there with my waders. It’s one of those things you’ve got to practice. No one’s good when they start out, but I’m really good at it now.

After I catch them, they’ll be transferred to the exam tub. I usually have a tub of their tank or their pond water that is ready to go with some sedatives. For most of my physical exams, I prefer that the fish be lightly anesthetized. It’s less stress for them; trying to restrain a wet, slippery torpedo is not really going to be in either of our favors. We just need them to be handleable. So they might be wiggling a fin at me, but once they’re anesthetized, I can get a really good look all around their body.

Usually we’re going to do skin mucus and gill biopsies. The skin mucus biopsy is mostly just looking for parasites, which can irritate fish and make them lethargic. The gill biopsies are more important because that can show us what their respiratory system is doing. It provides us a great diagnostic tool without having to stick a tube down their throat.

If we need to do any more diagnostics, ultrasound or radiographs, we can do that while the fish is asleep. One client has a pond of goldfish, and there’s one goldfish that just hasn’t been able to get up and swim with everybody; she’s kind of stuck on the bottom. We’re going to take X-rays.

A buoyancy disorder is when a fish that should be able to swim in the middle of the water column is either sinking to the bottom or floating to the surface. It’s very important for fish with buoyancy disorders that we can evaluate their internal anatomy — specifically their swim bladder, a little air sac that helps them float.

It can be diet related, too. This is very common in goldfish ponds, where you have fish that float after eating. If there’s a lot of competition and limited food during feeding time, it’s madness. They’re all just eating, eating, eating; they suck down a little bit too much air.

That can be corrected by spreading out the feeding a little bit more or feeding a sinking diet. A lot of fish diets float because it gives owners the opportunity to evaluate them at the surface, and obviously it’s a lot more interactive. Koi and goldfish are naturally bottom-feeding fish. But we have taught them — because they’re golden retrievers, they’ll do anything for food — to come to the surface during feeding time.

For fish surgeries, there’s many different levels. I do a lot of enucleations, which is eyeball removals. In fish, these are so easy; they don’t have eyelids or need to have any sort of globe in there to look normal. I did one for a little goldfish that actually had an abscess in her eye. The fish was just utterly miserable. We were able to get that out, and the next day the owner is, like, “She’s a completely different fish. She’s eating, she’s zipping around.” They heal up just beautifully every single time.

We see ovarian cancer very commonly in koi. If we catch it early enough, we can do surgery to remove it. We use a higher dose of the sedative. We have a specialized trough that the fish sits upright in. They sit over a little reservoir that has the anesthetized water. There is an aquarium pump that pumps it through a tube up into the fish’s mouth, it goes over their gills, runs down the side of their body and then back to the reservoir.

The biggest challenge is the public not even knowing that fish veterinarians exist. Even within our own profession, we’re ridiculed. Fish aren’t respected as pets in general. Like, “Why are you wasting your time? It’s just a fish.” For a lot of people, it’s not just a fish. It’s an actual living and breathing animal that needs to have care and respect. A lot of fish are taken into homes as practice pets and get the really short end of the stick.