Indeed, melosu58 is correct in that is alot of bioload for a 50 gallon tank. Some of the things that will determine the amount of fish you can keep will be what type of filtration and skimmer that you are running, along with what type of maintenance you plan on doing. Except for a few of the smaller species of puffer, they are not appropriate for that tank size. A small snowflake would be ok for awhile. Keep in mind,depending on the predator, it also will cut your options for a clean up crew.