Never put sucker-mouth fish with big fish unless it is bristle-nose pleco (sucking):
a) Goldfish with oto.
b) Back-skirt tetra with oto.
c) Goldfish with common pleco.
d) Goldfish with albino red-fin shark.

Never put fish with flowing fin with fish without flowing fin (biting):
a) Goldfish with carp.
b) Goldfish with tiger barb.
c) Goldfish with black/white-skirt tetra.

Never put fish with flowing fin with fish with long body:
a) Goldfish with any swordtail (normal or long-fin).

Never put big fish with small fish (biting):
a) Guppy with any swordtail (normal or long-fin).

Never put two species of fish with similar shape together (biting):
a) Guppy with male betta.
b) Long-fin swordtail with betta.
c) Small single-tail goldfish with albino red-fin shark.
d) Neon tetra with cardinal tetra.
e) Honey gourami with dwarf gourami.

Never put coldwater fish with warm-water fish (temperature):
a) Goldfish with platy.

Never put more than two small crustaceans with any fish about the size of the crustacean. The small crustaceans will group up to attack the fish. Anyway big fish (even double-tail goldfish) will eat the crustaceans.

Effect immediate
Never put big fish with small fish. The big guy will eat the small guy.
a) Big goldfish with guppy.
b) Neon tetra with goldfish.

Never put small fish with crustaceans, even small ones (eating):
a) Guppy with small shrimp (except cherry shrimp). Guppy will be attacked and eaten.
b) Neon Tetra with small shrimp (except cherry). Neon Tetra will be attacked and eaten.

Dwarf puffer will eat snail.
Dwarf puffer will pick on cherry shrimp.

Never put big crustacean with any fish. The big guy will eat the fish.

My planted community tank
Cardinal tetra (3), male honey gourami (3), harlequin rasbora (3), cherry shrimp (7), otocinclus (3), zebra snail (2), corydoras (2), bumblebee goby (2), guppy (2), red platy (3).