Tropical Fish Commands

This command will always spawn the same type of fish and the fish will never despawn (Which you probably don´t want to happen in your aquarium)

/summon minecraft:tropical_fish <position> {PersistenceRequired: 1b,Variant: <number>}

You can calculate your variant with this formula:

((256*<pattern color>+<base color>)*256+<pattern>)*256+<size>

or

  <pattern color> * 16777216
+ <base color> * 65536
+ <pattern> * 256
+ <size>

According to this the numbers for the pattern color and the base color are these:

0 for white
1 for oange
2 for maenta
3 for sky (cyan blue)
4 for yellow
5 for lime
6 for rose
7 for gray
8 for silver
9 for teal
10 for plum
11 for blue
12 for brown
13 for green
14 for red

The pattern has the numbers 0-5, left to right
The size is either 0, for the bottom row, or 1 for the upper row: