Any ideas of what to add?

My son and I have a 20g tank with 2 red-eyed puffers (irrubesco species), male and female, along with 3 emerald green cories and an algae eater. They all get along well, with no aggression except between the puffers when they are eating and that is only on occassion. I would like to add another fish or two to the tank but am not sure about what might get along with these guys. The tank is well planted and has several hiding places (caves). Each puffer has staked out theirs and the others share with them or use one of the other spots on occassions.

Thanks, have told my son no more fish for his tank. Although I have kept aquariums of various sizes, I have never kept puffers or cories. The algae eater is a Siamese Algae eater. When it gets larger it will be going into my husband's 55 gal tank at his office. It is mainly in the tank to take care of the algae that was going nuts. He did that and is keeping the tank fairly clean, am feeding him algae wafers every couple of days, too. We enjoy watching the puffers as well as the cories. So thanks for the advice and we will heed it.