i have a 20 gal with a blue gourami, 5 tiger barbs, and 1 rainbow shark with no problems.....for now atleast. with a large group i would say its safe for almost any fish except ones with long flowey fins such as angels, betas, fancy guppies, etc. gouramis, danios, most bottom/algea eaters, some cichlids would work.....this is just off the top of my head. just dont keep anyother fish in the middle section.........just an idea.....maybe you could have all tiger barbs and have a number of the normal, green, and albino ones. try asking in the correct section for barbs, tetras and such and you might get more accurate replies.