Humason, older edition, recommended sea water be used with the fixative,
formaldehyde in your case.  Your marine animals will probably like it
better since the salinity is important to maintain the isotonicity of
the cells ((yours blow up!).  I guess on could say the seawater is a buffer
in itself.  Once fixed, I don't know if you need to increase salt in your
immunostaining buffer.  This is is sometimes done with buffers, for IHC.

There is an artificial sea water recipe in this book, but requires some
interesting salts, etc to make it up.

Gayle Callis