Hey Guys,

Guide for what fish have special lot in the fish ponds:

Mudskipper
Soultrap
Void Evilpus
Goblin Shark
Decapod
Brain Slug
All types of Koi
Swordfish
Cavefish
Cave Lobster
Electric Catfish
Vampire Fish
Vampire Squid
Great Firefly Squid
Glassfish
Barrel Eye
All types of Goldifsh
Coelacanth
Spinny Dogfish
Cyclops Shark
Ladyfish (Requires Island Mod for Special Lot)
Octopus (Requires Island Mod for Special Lot)
Void Salmon (Requires Island Mod for Special Lot)
Void Carp (Requires Island Mod for Special Lot)
Blinky 
Bonnethead
Small Manta Ray
Black Ghost Knifefish
Clown Triggerfish
Arowana
Snakes (Both types)
Starfish
Dumbo Octopus

Notes: 

- Not all the items have the same chance to drop, so don't worry, nothing is going to be OP in here, if a certain fish has a chance to drop a very good item, be sure it's a tiny chance;

- Quests items required to increase the population are random picked in the list, you will not need every single one of the items to upgrade you pond, you'll only require the choosen one;

Watch out for spoilers now:

Mudskipper:]

Population 1 or above:
Mudstone 1
Population 3 or above:
Clay 1 - 3
Population 5 or above:
Wild Bait 1 - 10
Population 8 or above:
Artifact Trove

Quests:
Population 4:
Clay - 15
Population 8:
Snail - 5

Koi:

Population 4 or above:
Rice Shoot 5 - 10
Population 5 or above:
Celestine
Population 7 or above:
Jade
Population 10:
Cherry Sappling

Quests:

Population 4:
Bread 1 - 2
Rice 1 - 2
Population 8:
Tortilla 2
Corn 2 - 3

Swordfish:

Population 4 or above:
Roe 1 - 2
Population 6 or above:
Ancient Sword
Population 8 or above:
Bug Meat 20
Population 10:
Iridium Bar 1 - 3

Quests:

Population 4:
Haddock 5
Population 7:
Ghost Eel 5

All types of Goldifsh (Telescope, Lionhead,Celestial Eye, Commet)

Population 1 or above:
Roe 1 - 2
Population 4 or above:
Roe 2 - 4
Population 6 or above:
Roe 4 - 10
Population 8 or above:
Golden Relic
Population 9 or above:
Elvish Jewlry
Population 10 or above:
Golden Mask

Note: Yeah they produce a lot of roe

Quests:
Population 5:
Limestone 5
Population 5:
Slate 5

Electric Catfish

Population 5 or above:
Roe 2 - 2
Population 8 or above:
Battery Pack 1 - 2
Population 9 or above:
Battery Pack 3 - 5

Note: Don't know what you're gonna do with all these batteries but here you go

Quests:
Population 4:
Sandfish - 2
Population 8:
Battery Pack - 5 (that's just a joke, don't throw battery packs in ponds IRL)

Decapod:

Population 5 or above:
Muscle Remedy
Population 9 or above:
Energy Tonic

Quests:
Population 4:
Joja Cola - 15
Trash - 15
Broken Glass -15
Broken CD - 15
Strange Bun - 2
Population 8:
Pale Broth 10
Chocolate Cake 3
Strange Bun 3

Brain Slug:

Population 4 or above:
Fertilizer
Speed-Gro
Population 8 or above:
Quality Fertilizer
Deluxe Speed-Gro
Population 10 or above:
I Voted Sticker (Only In with Mermaid Island Mod) -  Not Implement Yet
Rare Seed

Quests:
Population 4:
Soggy Newspaper - 10
Population 8:
Gold Bar - 5

Goblin Shark

Population 1 or above:
Goblin Shark Pup
Population 4 or above:
Goblin Shark Pup 2
Population 6 or above:
Goblin Shark Pup 4
Population 8 or above:
Curiosity Lure
Population 10:
Blue Slime Egg 1 - 2

Quests:
Population 3:
Longfin Icedevil 5
Population 7:
Conger - 5

Spiny Dogfish

Population 1 or above:
Spiny Dogfish Pup
Population 4 or above:
Spiny Dogfish Pup 2
Population 6 or above:
Spiny Dogfish Pup 4
Population 8 or above:
Bone Fragment 10 - 20
Population 10:
Treasure Chest

Quests:
Population 3:
Anchoviella 10
Population 7:
Barracuda 5

Cavefish / Cave Lobster / Glassfish

Population 5 or above:
Geode 1 - 5
Population 9 or above:
Omni Geode 1 - 5
Population 10:
Treasure Hunter (the tackle) 1 

Quests:
Population 5:
Geode 5
Population 8:
Omni Geode 5

Note: Yeah invest geodes to get them back

Ladyfish:

Population 10:
Love Potion

Quests:
Population 4:
Frozen Geode
Seaweed 1 - 2
Driftwood 3
Population 6:
Coral - 2
Clam - 2
Population 8:
Ice Cream 2
Strawberry 5

Vampire Fish:

Population 1 or above:
Bat Wing 30
Population 5 or above:
Monster Musk
Population 9:
Golden Pumpkin 1 - 2
Quests:
Population 4:
Jack-O-Lantern 10

Population 6:
Small Goblin Shark 3
Spinny Dogfish 3

Vampire Squid

Population 0 or above:
Squid Ink 2
Population 4 or above:
Squid Ink 3 - 6
Population 4 or above:
Squid Ink 6 - 10
Population 8:
Pearl 1 - 2

Quests:
Population 4:
Clam 5
Population 6:
Starfish 10
Population 8:
Blue Starfish 4
Royal Starfish 3
Crown of Thorns Starfish 2
Holy Granade Starfish 1 - 2

Great Firefly Squid

Population 0 or above:
Squid Ink 1 - 5
Slime 40 
Population 4 or above:
Jasper 
Population 6:
Ruby 
Population 8:
Prismatic Shard 1 - 2

Quests:
Population 4:
Sand Dollar 5
Population 6:
Jellyfish 5
Prawn 3
Population 8:
Nautilus 1 - 2
Blue Dragon Slug 1

Dwarf Stingray

Population 1 or above:
Dwarf Stingray Pup
Population 4 or above:
Dwarf Stingray Pup 2
Population 6 or above:
Dwarf Stingray Pup 4
Population 8 or above:
Rusty Cog
Dwavish Helmet
Population 10:
Dwarf Gadget

Quests:
Population 3:
Coral 10

Dumbo Octopus

Population 4 or above:
Nekoite 
Population 6 or above:
Gold Bard
Population 9 or above:
Omni Geode 1 / 3 / 10

Quests:
Population 3:
Coral 3
Honey 1
Oyster 1
Refined Quartz 3
Population 5:
Dried Starfish
Emerald 2
Omni Geode 2 - 3
Purple Mushroom 2 - 3
Population 7:
Green Tea

Starfish (all types)

Population 10:
Starfruit Seeds 3 - 10

Quests:
Population 4:
Cockle
Clam
Population 8:
Oyster 5
Snail 5

Snakes (Sea Snake / Water Snake):

Population 5 or above:
Snake Skull
Snake Vertebra
Mummified Bat
Mimmified Frog

Quests:
Population 4:
Blue Discus
Black Ghost Knifefish

Soultrap / Void Evilpus:

Population 0 or above:
Roe 1 - 2
Population 1 or above:
Fairy Stone
Population 8 or above:
Fairy Dust 
Life Elixir
Population 10:
Magic Rock Candy

Quests:
Population 4:
Purple Mushroom 10
Population 6:
Solar Essence 50
Population 8:
Fairy Dust 5
Population 10:
Prismatic Shard

Small Manta Ray:

Population 1 or above:
Manta Ray Pup
Population 4 or above:
Manta Ray Pup 2
Population 6 or above:
Manta Ray Pup 4
Population 10:
Purple Slime Egg 1

Quests:
Population 5:
Coral 20
Population 9:
Rainbown Shell - 5

Coelacanth

Population 2 or above:
Cave Carrot 5
Population 5 or above:
Nautilus Fossil
Population 6 or above:
Amphibian Fossil
Population 8 or above:
Dinosaur Egg
Population 10:
Artifact Trove

Quests:
Population 3:
Stonefish 5
Population 9:
Cave Lobster 3

Arowana:

Population 2 or above:
Roe 1 - 2
Population 4 or above:
Roe 2 - 2
Population 10:
Prismatic Shard
Quests:
Population 3:
Red Freshwater Shrimp 5
Blue Freshwater Shrimp 5
Population 9:
Rainbown Trout 5

Clown Triggerfish

Population 1 or above:
Roe 1
Population 4 or above:
Mango 1
Population 8 or above:
Mango Sappling
Population 10:
Warp Totem: Island 5

Quests:
Population 3:
Banana 2
Population 9:
Golden Coconut

Black Ghost Knifefish

Population 1 or above:
Roe 1
Population 4 or above:
Ginger 1 - 3
Population 8 or above:
Rain Totem 1 - 2
Population 10:
Qi Seasoning

Quests:
Population 3:
Freshwater Shrimp 5
Population 9:
Magic Bait 5

Bonnethead:

Population 1 or above:
Bonnethead Pup
Population 4 or above:
Bonnethead Pup 2
Population 6 or above:
Bonnethead Pup 4
Population 8 or above:
Cinder Shard 5 - 10
Population 10:
Tiger Slime Egg 1

Quests:
Population 3:
Pufferfish 5
Population 7:
Super Cucumber 5

Cyclops Shark (doesn't produce roe nor reproduces):

Population 4 or above:
Geode 10
Population 8 or above:
Omni Geode 10
Population 10:
Artifact Trove 5

Blinky

Population 0 or above:
Roe 1 - 20 (yeah mutant fish)
Slime 1 - 5
Population 4 or above:
Slime 3 - 8
Duff (Beer ) - Not Implement Yet
Population 10
Radioactive Ore 1 - 3

Quests:
Population 9:
Radioactive Bar

Octopus
Keep vanilla lot but at 10 you can gain up to 2 Takoyaki

Void Salmon
Keep vanilla lot but at 10 you can gain a Void Poison

Void Carp
Same rules as Void Salmon