Need ideas for addition of fishes

[onlyyouknow]

Hi guys,

I currently have six 3x2x1.5 tanks and would like to add more fishes
Here are the current setup:
Tank 1: one lungfish size about 10-12 inches
Tank 2: turtle size 5 inches
Tank 3: planted with endlers and Galaxy rasboras
Tank 4: 11 pieces of golden red discus
Tank 5: shellies tank
Tank 6: one crayfish and some small fishes

Any suggestions on what I can add?
Probably not to the lungfish and discus tanks.