​Is the routine use of potassium permanganate (PP, or Condy’s crystals) harmful to your fish? 

A lot of the Koi people routinely use potassium permanganate for disease control, and to clarify water. Similarly, those growing fish for food production may use it to clear up superficial bacterial infections. It is an oxidising agent that is basically used at a small enough dose rate that’s enough to zap anything that’s small, and leaves the fish’s skin and gills relatively unharmed. 

But does it do no harm to your fish? A lot of people swear by its routine use, and many have experienced no adverse effects. Maybe they’ve just been lucky? 
Our The Fish Vet team member, Dr Giana Bastos Gomes, brought this article to our attention. 
A recent publication by Mohammed & Arias in the journal of Veterinary Research (2015) show that PP treatment dramatically altered the community variety of normal resident bacterial flora on the skin of fish that have been exposed to PP – they had the least diverse microbiome in terms of species richness.
Why is this bad? They also ran an experiment where they challenged fish with a common bacterial skin disease agent – Flavobacterium columnare (previously Flexibacter, causes fin rot,  gill rot and bacterial ulcers in fish). The deaths in fish  previously exposed to PP was about forty-percent (40%) worse (85% cumulative mortality compared to unexposed groups that had a mortality of just over 60%).
Just like people know that we ought to line our guts with healthy Probiotics bacteria, having a healthy range of bacteria on the skin of fish is protective for fish, possibly because of competitive inhibition of pathogens, or through immunoregulation.
This might explain why some fish keepers have commented that their fish thrived in less than ideal waters, but then when they clean up everything and do as they’ve been told by fellow fish-keepers, their fish die.
Read more of the article,“Potassium permanganate elicits a shift of the external fish microbiome and increases host susceptibility to columnaris disease.” by Haitham H. Mohammed and Covadonga R. Arias.

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