Many cichlids breed in groups consisting of one male and one or more females. The challenge for the home aquarist is to establish these breeding groups in a single tank. Oftentimes, this involves either purchasing adult fish, putting them in the same tank and hoping for the best, or raising a group of juveniles and hoping for the desired male/female ratio. Due to the extreme expense of purchasing a large group of adult fish, aquarists most often raise juveniles. The issue with raising juveniles is that it is often difficult, if not impossible to determine the sex of many cichlids while they are young. In addition, some uncommon or highly desirable juveniles can still require a substantial cash outlay (and even more, if purchased through mail-order).
Often, the ideal male/female ratios published are in the range of 1m/2f to 1m/4f. The idea is that no single female will bear the brunt of an aggressive male's harassment. The more females around, the more the aggression is spread between them. However, depending on the species, and the temperament of the individual fish, this ratio may vary. So with that in mind, how many fry should one purchase?
The answer can be determined from a simple statistical analysis. The table below was generated from a Monte Carlo analysis whereby a computer program generated 50,000 random sets of either 4,5,6,or 7 fish, and the male to female ratios were simply counted up and are expressed as a percent probability. The table shows values of "at least" that male/female ratio occurring. For example, it is 62% likely that you will have at least 1 male and 2 females out of 4 fish. You could have 2 males and 2 females, or you could have1 male and 3 females, but the overall probability of you having your desired 1m/2f ratio is 62%. The assumption is that you would trade/sell/cull the unwanted males or females.