Aldehydes (formaldehyde, PFA) are standard histological fixatives. Immersion of the larvae in 4% PFA in 0.1M PBS (pH7.2, ~200 Osm, made fresh) overnight or longer at 4deg C should be sufficient. Use at least twice the volume of solution as your specimen(s). Make sure the larvae are euthanized just prior to fixation to reduce artifacts. Because the fish are very small you should not need any additional steps (e.g., flushing of fixative into buccal cavity, incisions in ventral body wall) to help with perfusion of the fixative.

Hi. Thanks Pramod for that reply and I have been using NBF (neutral buffered formulin) for the larvae just as you state, 24 hours at room temp works very well for them. It was the specific use of Hartman's fixative (modified Davidsons') if anyone has used that on 4dpf ZF larva. It was a recomended by Wolf et al 2015 as a better fixative for fish. This below is the current NBF fixation (40x mag heart tissue).