Remove formalin from food using chemical found in kitchen
While the idea of filtering formaldehyde from food might sound naive, the motivation is understandable and the problem apparently real.

I've found at least one publication on the topic: 

T Yeasmin, *M S Reza, M N A Khan, F H Shikha and M Kamal

PRESENT STATUS OF MARKETING OF FORMALIN TREATED FISHES IN
DOMESTIC MARKETS AT MYMENSINGH DISTRICT IN BANGLADESH

Int. J. BioRes. 2010, 1, 21-24

From the abstract:


  [...]Study showed that formalin was not detected in any
  fish produced locally, but was detected in the imported ones of catla and rohu ranging from 0.5% to 1% which was
  sold in different markets of Mymensingh with comparatively lower price than those produced locally. The shelf life of
  the locally produced fish was much longer than those of imported fish which may be related to the loss of shelf life
  during its transport from importing countries. [...]


Concerning the initial question:
I'm afraid but i don't see any chance to remove formaldehyde from food without turning it to another toxic cocktail.

But since formaldehyde was mostly used to increase the shelf time of imported fish, buying from markets predominantly selling local products might be a way--if available and affordable.