Whilst that's a useful guide, it doesn't take into acount the numbers you are keeping, the size of the tank or the sex of the individual. There are many mroe factors, besides species, that affect compatibility. Take, for example, tiger barbs - a group of 10 may be fine in a tank with a red tail black shark - now take out some of those barbs, leaving a group of 3, and they become extremely nippy and constantly harass the black shark - one day he turns on them and kills them. Simialrly, think of the difference in compatibility between male and female bettas or consider how much easier it is to keep a group of 20 malawian cichlids of various species compaired to a couple of males from different species (incase you don't know, the former is common practice, the latter would typicaly result in the death of at least one of the two). Even fish considered 'peaceful' vary in their compatibility. Look at male guppies for example - in a 20 gallon, a couple of males should be fine but put them in a 5 gallon and they can actualy become quite nippy towards each other.
/www.liveaquaria.com/general/fwcompatibility_chart.cfm B)
