How many Frontosa's

Imo a 150 is marginal for a Frontosa colony for the reason Pappy stated. You should keep them in as large a group as possible with a great preponderance of females. Six would be my minimum group as well, 1.5 or maybe 2.4 Six full grown Fronts is alot of fish for a 150.
 
At least 2 males and 4 females. If there is no competition among males there may be no attempt at mating. Mating is a competition for the alpha male to pass on his genes. Fish aren't stupid nor do they breed just to breed.
 
Fish aren't stupid nor do they breed just to breed.

Males breed all the time with no other males in the tank. Breeding is a survival tool, not just a competition, and an instinct that all fish are born with. Say if a species was on the brink of extinction, and there was only one male in an area, they would still need to breed. You're right that they breed to pass on their genes over other males' genes, but this only needs to happen if there is competition, which is not necessary in the wild or a fish tank.
 
As for stocking, I agree with Pitt. A 150 is big enough for two males to coexist.
 
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