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RE: malachite green

> Date: Mon, 29 Sep 1997 00:12:36 +1000
> From: Ross Drewe <rdrewe at melbpc_org.au>
> Activity: (1) fungi (2) some Gram-positive bacteria (3) proliferative 
> kidney disease in salmonids (thought to be myxosporidial).
> Usage: As mentioned: fungicide, topical bactericide, dye for yarns. In 
> aquarium use, most commonly as a bath (ie, added to the aquarium at 0.1 
> mg/L or less) but this is often ineffective because of the relatively 
> low concentration.  
> Cautions: (1) Tetras and scaleless fish more sensitive. (2) Some plants 
> sensitive (someone please help with more details!) (3) Teratogenic, 
> therefore not permitted on food fish. (4) Stains everything in sight, 
> including plastics.

Old stuff, new uses - Malachite green is one of the dyes used in photodynamic
therapy, a relatively new cancer treatment.  These dyes have the property of
forming highly active (toxic) derivatives when exposed to light.  THe patient
is given the compound which is taken up by the cells.  Then a light fibre
is used to direct intense light of appropriate wavelength to the cancerous
areas, which are selectively killed.  THe patient has to stay out of direct
sunlight until the dye clears from the body.

It's been used for early cancers of the esophagous for a few years, and 
looks promising for early bladder cancers too.

I've wondered whether it works by the same mechanism in the aquarium, and
whether the fish scales are protective from light, since scaleless fish
such as loaches and some catfish are killed by doses that don't affect
other fish.  Possibly small or relatively transparent fish like many tetras
are harmed this way?