[Zbrafish] Zebrafish Embryo Fixation

[Harry]

Dear friends,

I am working in Zebrafish and I am a beginner. My interest is in whole
mount ISH. I am suppose to make an atlas of developmental stages of
the embryos at first as a part of standardization of fixation and etc.
I am trying to do a small scale experiment. I put 3-4 embryos at
particular time intervals in 150 microlitres of freshly prepared 4%
Paraformaldehyde. After keeping it in 4 degrees  for 8-10 hours, I try
to take the photograph of the fixed embryo. I put the embryo in a
petridish with 1X PBST and dechorionate that under a stereomicroscope.
The embryo turns black (especially the yolk) after dechorionation. If
further disturbed, the yolk disintegrate into small cell like
structures. I would like to know the reason for the happening and
humbly ask for suggestions to correct or improve my procedure of
fixation.