How to set up your tank fish from top to bottom
To increase the chances of your fish being compatible you need to consider the fishes’ territory. Fish swim in different zones of the aquarium. Some swim near the surface. Some swim midwater. And yet others stay near the floor of the aquarium. Generally fish in one zone will be less territorial with fish from another zone.
Here I will list suggested fish groups by tank size with an eye on maximum compatibility.
2 foot 50litre tank

Small amazonian biotope
Surface fish
4 hatchetfish
Midwater fish
8 neon tetras
2 angel fish
Bottom dwellers
4 panda corys
Small slice of an Asian stream in your living room

Surface fish
4 hatchet fish
Midwater fish
4 Harlequin rasboras
6 cherry barbs
Bottom dwellers
4 kuhli loaches
1 siamese algae eater
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Dwarf Cichlid Breeding set up
Surface fish
4 Lemon tetras
Bottom dwellers
Male and 2 female cockatoo dwarf cichlids
3 three stripe corydoras
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Livebearer community set up
Surface fish
6 guppies 2 male 4 female
Mid water fish
3 sword tails 1 male 2 females
Bottom dwellers
3 bronze corydoras
More on livebearers here
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30 inch 76liter tank
Amazonian biotope aquarium
Surface fish
2 silver hatchet fish
midwater fish
3 emperor tetras 1 male + 2 females
5 lemon tetras
Bottom dwellers
3 peppered corydoras
More on the Amazon biotope here
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Slice of an asian stream
Surface fish
3 glass catfish
midwater fish
6 harlequin rasbora
Bottom dwellers
5 kuhli loaches
1 siamese algae eater
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Amazon dwarf cichlid aquarium
Surface fish
6 cardinal tetras
midwater fish
mated pair(1 male+1 female) of blue rams or bolivian rams
Bottom dwellers
6 pygmy corydoras
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4 foot 110 litre tank

Large gourami Asian biotope
Surface fish
male and female pearl gourami or male + female blue gourami (breeding pair)
midwater fish
6 checkerboard barbs or 8 harlequin rasboras
Bottom dwellers
5 kuhli loaches
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Asian busy community tank
Surface fish
5 zebra danios
midwater fish
5 rosy or ruby barbs
Bottom dwellers
2 siamese algae eaters
6 kuhli loaches
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Non-rift valley African community tank
Surface fish
midwater fish
5 red eye tetras
Bottom dwellers
5 kribensis
More on kribensis here
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Convict community
Surface fish
6 guppies
midwater fish
5 bleeding heart tetras
Bottom dwellers
5 convict cichlids
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Malawi Community
Surface fish
3 Mollies
midwater fish
6 peacocks (jacobfreibergi) 2 males 4 females
Bottom dwellers
6 yellow labs