I don't have much experience with fishes and don't know the size of a killifish, but in my opinion it should work. In most cases, at least if you want to perform immunofluorescence after your fixation, we have a tendency to over-fixate rather than "under-fixate". The method you propose was even used with whole mice heads, even if it didn't give beautiful fixations. The only problem I see with it is the persistence of red blood cells that could interfere with your posterior analysis, but then it depends on what you want to do...

Hi Siri,

I am not familiar with the specific fish you mentioned. However, I was looking into a method to fix adult zebrafish brains without perfusion. I found this JOVE protocol (see link) that might also be usefull for you. I didn't get the chance to try it yet but it looks pretty good. As Fabrice mentioned a possible problem is the presence of fixed red blood cells in your sample, in this protocol they include a 5 minute bleeding period (in PBS) to solve this.

Cheers,

Pieter

http://www.jove.com/video/51753/stab-wound-injury-zebrafish-adult-telencephalon-method-to-investigate