This is the most realistic aquarium simulator out there and very easy to design and I absolutely love displaying my aquariums on my tv and keep it on all day. Now, some friendly suggestions I came up with :)

TECHNICAL
-Be able to click a button to clear all fish out of aquarium, or all decor.
-When duplicating a tank it would be nice to be able to duplicate just the tank and not the fish.
-Button to click that instantly snaps you to front 2D view.
-Undo button
-Ability to hold in a button and be able to rotate with track pad.
-Key binding options.
-Total sandbox mode where you can mix fresh and saltwater everything (but that is only because I really wish there were saltwater plants to choose from)
-Type in species in search box to find things quickly.
-Organize fish species with filters
-A list to see all fish you currently have in your aquarium.
-Ability to clone decorations and plants
-Enable modding ability on Steam

DECORATIONS
-Leafy saltwater plants please!
-Floating plants like duckweed
-Log caves
-Cave decor
-Horizontal bars of bubbles
-Black sand
-A fake castle with tunnels and holes fish can swim in and out of
-Lava stone
-Flat thin rocks so you can build bridges and stack them

ANIMATIONS
-Shy fish that like to hide in one spot under a cave or rock or under plants
-Clownfish hiding in those anenomes like they do
-Ability for some fish to follow your cursor out of curiosity would be a really cute touch!

SPECIES DESIGN
-It would be awesome to have presets of actual fish species colors: like different color guppies, Bettas, Discus. The randomization ability gives very unrealistic color variations. For example, you have a Tiger barb and one of the presets is the Albino Tiger Barb and another preset is a lighter/paler Tiger Barb, etc.
-Male/female color variants
-Different sub-species of a fish (i.e. Kissing Gouramis, Moonlight Gouramis, Dwarf Gourami, etc.)

SPECIES I WOULD LOVE TO SEE
-Snails
-Hermit crabs!
-Crayfish
-Starfish
-♥♥♥♥♥♥ Loaches
-Seahorses
-Balloon Molly
-Longhorn Cowfish
-Different Goldfish species
-Eels
-Pipefish
-Lionfish
-African Butterfly Fish
-Moorish Idol
-Paradise Fish: blue and albino
-American Flag Fish
-Green Tiger Barb
-Albino Tiger Barb
-Melon Barb

Again, really love the game!