This is not a community fish, and is best kept in a species setup. It will relentlessly attack any silver-coloured or shiny tankmates, stripping them of scales and fins very quickly. Even larger, predatory species are not safe. Scaleless and non-reflective fish are generally left alone. Success has been had keeping Exodon alongside various loaches, Loricariids and some larger characins such as Anostomus. The situation will always be somewhat unpredictable though. It's best to add the Exodon last as they may see any new additions as food, and as with most predatory species they tend to bite first and ask questions later.
Make sure you buy a sufficiently large group of these. A minimum of a dozen is usually recommended, but buy as many as the tank can house. In a tank like the one described above a group of 25-50 would not be an unreasonable number. If kept in smaller groups, they tend to pick one another off until only a single fish remains. In large shoals, no individual can be targeted and it is usually only sick or unhealthy specimens that are killed, although occasional losses should still be expected. The interaction of a big group is fascinating to watch, as they squabble amongst themselves constantly. Feeding time in particular is quite a sight. Buy the entire group at the same time as new additions are usually attacked.
