Probably will be a DUH moment to many of you, but it was news to me...
I have several pairs of rainwater killies collected last September in full saltwater. I have them spread between my large saltwater tank and my large freshwater planted tank. Back around Christmas I found 3 fry in the canister filter for the saltwater tank. I rescued them and they have thrived, already looking and acting like small adults.
That accidental success led me to believe breeding them was gonna be easy. Observation of lots of courtship behavoir in both tanks made me think I was homefree. I even saw females slinking out of clumps of plants with guilty looks on their faces followed soon after by males with big grins.
But no fry! I couldn't understand it. Then something unusual happened.
I'm raising up a bunch of SRBD fry in a 20 long. I have periodically removed bunches of plants from the planted tank and thrown them in with the dace fry so they can hunt for little critters.
The other day I was moving plants around in the fry tank and saw a couple of fry that were obviously not dace. I recognized them as rainwaters but they acted kinda weird. They were perching in the floating plants and when disturbed, seemed to swim funny and insisted on perching rather than swimming free. Once I looked closer, I realized there were several more down in the gravel and they were acting even weirder. They hopped around on the bottom just like darters. I had no clue what the problem was.
Then I got some flag fish. Researching flag fish I came across an article in which the author described poor spawning success when the water was too deep for the fry to reach the surface to inflate their swimbladders. In a ten gallon tank!
AHHH HAA!
I immediately thought of all the banded killies and mummiechugs I've seen spawning in the wild - in 6 inches of water.
So anyway, now that I've made this a novel , I'm off to set up a tank with six inches of water in it.

I wonder if this would explain why my sheepshead minnow fry never survive long...

I wonder if this would explain why my sheepshead minnow fry never survive long...

I bet it is.
The article where I got the idea was about flagfish. Flagfish are about as close to sheepshead as any fish I can think of.
Also, in the wild I frequently observe male sheepshead displaying in water barely deep enough to cover them. I'm talking two or three inches at best. In fact, that would be at high tide so any eggs deposited might actually be left high and dry.
There is definately a shallow water factor at work here.
I've got rainwaters fattening up in a four inch deep tank. Hopefully soon I can report success, although there doesn't seem to be much interest in these drab colored little guys.

Mike, I have a group of rainwaters and was wondering if you've figured out how to successfully spawn them yet in FW. They are currently in a community tank but would like to set something up for them.

Well I know someone who has a male and female black molly and it has been hard for them to breed. So maybe this is the key, I will pass the word along. Thanks.

I guess I missed this back in August.
Rain waters spawn very well in freshwater.
Shallow water, clumps of plants or spawning mop, typical killie breeding.
Remove the plants/mop to a hatchery tank. Fry are large enough for baby brine shrimp and will take powdered flake food.
Sorry this is sparse, I'm pressed for time.