Hello,
Welcome to the hobby
Well, I'm sure you've done some research so I'll cut to the chase.
You may add pencil fish/ hatchet fish (I'm sure many sources have suggested these) for they would help to disperse the aggression with the apisto. Furthermore, if they do breed, they have a lower probability in targeting fries as they hang around the top surface while apisto are usually on the ground.
With invertebrates, you can try nerite snails as i doubt your tank is lower than pH ~6.5.
Anyway, are your fishes currently fighting? By now I'm sure you know that they are territorial, and I'm not entirely sure if 20gallons would work for 3 pairs, especially if they are about to breed. You would have to ask yourself if you would want to breed them. If you do, a separate tank for each pair would be best (maybe alone or coupled by some other fishes to disperse the aggression towards each other while breeding). But you have to consider if you would be able to house 3 pairs separately, and eventually their babies in their own tank as well.
If you don't intend to, and if your fishes are fighting a lot, provide them with more cover (plants, dried leaves, caves) and they may fight less. If they do breed with all 3 pairs in a tank, there is a higher chance for the babies to survive to adulthood as well. But do consider the bioload as well with more fish in the form of adding more tank mates or if they breed.
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