The best replacement for formalin is formaldehyde.

The best alternatives to formalin are rated as following:
★★★★★  formaldehyde
★★★  paraformaldehyde
★  malachite green
  4 paraformaldehyde
  formalin

Your alternative for formalin is not listed? Suggest!
Speaking about formalin, one probably shouldn't forget about malachite green.

Other search terms related to formalin and their ratings are:
★★★★★ malachite green
You have in mind a companion for formalin and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of formalin in all years. Click on graph for more.
formalin
formaldehyde
paraformaldehyde