If I don't get a caviar at the traveling cart, I do sturgeon. Once when I couldn't seem to catch one, I bought one and put it in a pond to fill the bundle and get the caviar!
I've also done crabs for the pearl, and then sold the crabs after.
If there are certain fish you want for recipes, for fishing, speed, or luck buffs, or to give as loved gifts, the fish pond is great. Sea cucumbers for lucky lunch, tuna for fish tacos (Gus & Caroline's favorite), rainbow trout for trout soup, eel for fried eel (Jodi loves) or spicy eel (Abigail loves), sardine for dish o the sea, flounder & midnight carp & squid or midnight squid for seafoam pudding (plus calamari is Pierre's favorite), clams or lobsters for chowder or bisque, shrimp for cocktail, crabs for crab cakes (Elliot loves), snails for escargot (Gus loves), crayfish mussel & periwinkle for fish stew, Alex's salmon dinner.
Abigail loves pufferfish, Penny loves sandfish, Elliot loves lobster, wizard loves super cucumbers, Vincent loves snails, Willy loves catfish sturgeon octopus and sea cucumbers.
The wiki page about fish ponds gives the details about what each fish might request, and which rewards they give. Some of my favorites have been ice pip, ghost fish, spook fish, blobfish, stone fish, woodskip, and rainbow trout.
Since each fish pond takes 25 tiles, same as a full iridium sprinkler, they are not great money makers. But there are fish that will add value to your game overall, by giving buffs or helping you build friendships or collect useful items, or just increasing your enjoyment of the game!