Hello,

I agree with Bozidar's answer. We also work with adult fish around that size (zebrafish, medaka, killifish). If you are interested in the gut and would like to section the entire fish, then penetration of the fixative into those deeper tissues is big concern. We use 10% neutral buffered formalin or 4% paraformaldehyde to fix unless there is something specific that will be done with the sections other than H&E (or similar) staining. 

Here is what we do:

1. Make a midline incision through the ventral body wall extending from the anus to near the pectoral girdle. Make sure to only cut through the skin and muscle layers, taking care not to puncture any organs (heart, gut, etc.).

2. Using a disposable transfer pipet with an extended fine tip (e.g., https://us.vwr.com/store/product/4758781/disposable-transfer-pipets-fine-tip-samco-scientific), introduce fixative to all of the abdominal organs through the incision.

3. Insert the pipet tip into the buccal cavity and flush fixative into the pharynx and proximal digestive tract.

4. Immerse the whole animal in fixative at room temperature overnight (or longer).

Fish will probably need decalcified prior to processing for sectioning - usually with 10% formic acid/EDTA for 48 hours.

When we follow these steps, we have found good fixation of the gut and surrounding tissues, and we have not had problems with the air bladder. Sagittal/parasagittal orientation should give you a good view of the gut. Paraffin embedding is relatively inexpensive and gives a fair amount of resolution. If you have the funds, other techniques will provide much greater resolution (e.g., glycol methacrylate).

Also, we will sometimes remove the tail (cut caudally to abdomen) during fixation so that we can fit more sections on each slide -- this will depend on the size of your fish though.

Removing the gut should just require you to immerse that organ in fixative and then process and section. Care should be taken to keep it as intact as possible, it is easy to stretch and rip. Because it is smaller, you will be able to orient it as you wish (as opposed to the coiling/folding in situ) and fit more sections per slide -- just keep track of which end is proximal and which is distal to the buccal cavity. However, you will miss surrounding tissues (liver, pancreas, etc.).

Hope that helps!

Best,

Melissa

http://www.oecd.org/env/ehs/testing/Histopath-Guidance-Document-for-the-Medeka-Extended-One-Generation-Reproduction-Test-Part_1.pdf