Molecular Probes has a useful tips for compatibility of alexa dyes with
antifade reagents and mounting media.  With your particular Alexa, they
do not have it on the list but you could contact them to see if it has
been added. This is a nice comparison for photostability and relative initial
fluorescence intensity using 6 different media.

To have best relative initial fluorescence with some of the Alexa's
PBS is consistently the best.  AND not as nice to use at times, but
we opted for it with Alexa 488 and 546, sealed the coverslip with
Richard Allan mounting media, toluene based, diluted to the consistency
of fingernail polish, in fact dumped the polish and used that container,
in order to avoid the isopropyl alcohol in the fingernail polish.  Our
fear was leaching of the isopropyl into the PBS.  It does require
some fast viewing as it does dry out.  

Interesting you have to use such a low dilution (high conc) of the dye,
we were diluting these out on PFA fixed frozens 1:500 and 1:1000 for
acetone fixed sections or we had major background fluoresence.  

Keep us posted

Gayle Callis