Thank you for the information. I'd vaguely heard about this sort of situation but had never come across an actual example. I have been syphoning out the debris at the bottom of the tubs and sieving it to try to find if the females have been scattering their eggs but have not found any. Last year the red/black females attached their eggs to the sponge filter but I have not seen any this year. In fact I have been checking the fish every morning but have not seen any females carrying eggs therefore I assume the medaka have not gone through the spawning process. I'm hoping that changing things to 1 male and a few females will stimulate the process. A good idea though to place a spawning mop on the bottom of the tubs to see if that helps. I may also add a mop made of knitting yarn to see if the females prefer that. Thanks again.

Thanks, maybe you only crossed your fingers on one hand ..... had eggs for 3 days from the yozakura ..... but none from the red/blacks. So only leaving one male in a breeding group is an option worth trying. I mustn't complain too much ..... but only 1 female yozakura is producing eggs and unfortunately this female is probably the least well-colored of the group of yozakura females. Just for information ..... it must be the first time this female has produced eggs. Day 1 was 2 eggs; day 2 was 3 eggs and then 12 eggs on day 3 ..... however the female has not yet got the idea of depositing eggs one at a time spread out on the spawning mop. The 12 eggs were deposited in one place like a bunch of grapes. Interesting how eventually females instinctively start to deposit eggs individually.