Baby Bristlenose Dying ...strange Bellies

[mitmop]

Hiya all. not sure this is the right place to post but I have bred baby bristlies for a few years now with no problems. I have recently over the past month have lost about 40 + babies. They are developing black patches on the belly and then just dying. They dont seem in distress or anything. It only seems to affect the small ones no bigger than an inch. They are in the fry tank and non of the other fry (cories or guppies) have died. it only seems to be affecting the bristlies. Has anyone seen this before. They seem to have red tummies or black patches. They are eating fine and moving around etc. This has baffled me

if you have any ideas about this or have seen it before would be grateful of suggestions.

thanks

dawn