This is going to sound crazy but you do not have enough Molliro to put in a 90 gallon. They are very aggressive toward each other and need to be over crowed to survive in your tank.
You will notice one dead, then another until you have just one. Either get 30+ more or put them (at the risk of getting flamed) in a 30 gallon and over filtrate.
It's also very easy to id the males but it takes time. Watch the tank over time and remove the one that flashes or shimmers sideways (only one should be doing this at a time). 99% of males do this and 99% of females don't.
The problem with this method is you will never know how many males you have until they are all gone and 2 or more will not survive in a community tank. It's a tough fish to breed because of their temperament and water conditions. Good luck
Here is some info from cichlidforum.com:
Conspecific Temperament: Highly Aggressive
Maximum Size: 5"
Temperature: 77 - 79°F
pH: 8.6
Water Hardness: Very Hard
Difficulty: 4
Conspecific Temperament: This describes the overall demeanor of a cichlid toward other tank-
mates of the same species. Consider that there is variability in temperament due to such factors as
aquarium size, stocking levels and order of introduction. There may even be some variability among
individual specimens.