Warning! Formaldehyde can be very toxic in aquatic environments

[Lasse]

I have understood that some fish "medicines" can still contain formaldehyde. I thought it not was in use any longer. Besides effects on humans (supposed cancerogenic and other effects) it can convert to a very toxic substance paraformaldehyde with time, especially toxic for fish. It is not question if – the question is when? Old batches can have formed paraformaldehyde and if it have been stored below 5 degree C - even fresh formalin can convert to the toxic form.
However – there is a chance to see this. Good formaldehyde is a clear liquid without any cloudiness – if you can see precipitation in one or another way – paraformaldehyde has been formed. If the bottle contain formaldehyde and show precipitation DO NOT USE IT

More reading here

Sincerely Lasse

 

[ngoodermuth]

I’ve yet to touch the stuff. I’ve been able to find a suitable alternative in every case so far, and generally discourage using it whenever possible.

This is a great heads-up regarding the medication “turning” to the more toxic form.... I honestly didn’t even realize that could happen until you’d mentioned it in another thread.

 

[Big G]

Have one of the last bottles of Formalin. I avoid it unless drastic measures are needed to save a fish. The possible side effects always give me pause.

 

[Lasse]

Be carefull - old - probably some paraformaldehyde in it. If you should use it look very carefully for precipitation. If it have that - away with it.

Sincerely Lasse

 

[rkpetersen]

I have formaldehyde/formalin 'on hand', but have never used it on fish.
When I was a kid and very interested in microscopy, I used gallons of formalin to preserve specimens, lol.
And then I worked in an autopsy lab for a while and the entire place smelled of it.
I even rather liked the smell.
Was also very interested in chemistry when younger and frequently used carcinogenic benzene as a solvent.
No cancer yet. ymmv.