I wanted a bunch of fish ponds on my farm, and have 8 currently. I like how they look, but kinda regret using so many different types of fish. Because I run into practical problems with inventory space when picking up all the different stuff each morning, while running around and stressing about checking sheds for machines that need filling. Sadly the roe for most of them isn't worth very much :sad:
Anyway, these are the fish that I'm using.
Blobfish
Lava eel
Void salmon
Ice pip
Sturgeon
Super cucumber
Octopus
Eel
The point about Eel was to get ingredients for Spicy Eel, but as it turns out, with the Burglar's Ring, I'm getting enough drops here and there anyway, and with all the stress on the farm, I've never been back to Skull Cavern after getting to level 104 that one time (for the quest). I picked these fish for the somewhat rare unique-ish drops.
Blobfish - Very rarely pearl, roe is worth a good chunk
Lava eel - Magma geode and Spicy Eel, and the roe is worth a good bit. Main thing is the beautiful red water :D
Void salmon - Void egg (but rarely)
Ice pip - Many different drops: diamond, iron ore, frozen geode, frozen tear
Sturgeon - Caviar when processed
Super cucumber - Iridium ore (low chance, but still), amethyst
Octopus - Omni geode, but roe isn't worth much given how darn difficult these are to fish up
Eel - Only produces roe, so the main benefit was the easy access to the eel ingredient for Spicy Eel. Which turns out wasn't needed :D