What is this critter??

[FISHNUTS]

Specs 25 gallon tank with 4 L136 and 5 L-08 and 4 Kamaka Rainbows. Yes it quite a few fish but it is over filtered and I change a lot of water. I do have about 20 tanks in all and been keeping fish for some 20 years.

What is this critter?




I treated the tank with Potassium permanganate. Brought the tank down to 50 litres and used .142 dry grams of PP for 30 min. The water was yellowish/brown when done instead of bright purple.

Thought this may have killed them but after the treatment I have seen these come out at night darting away from the light. How I noticed something was wrong in the first place was the pleco's were all hanging around the top part of the aquarium. Ok at night but they were doing it under the full lights.

Did I use the right measurement to dose?

Thanks
sl

[Bully]

I'm not 100% certain, as I don't have first-hand experience, but I believe it's a fish louse, Argulus.

[Diana]

If PP turns non-pink too soon then it needs to be redosed.
It combines with organic matter in the system. If there is a lot of fish waste, fallen food or many other things then the PP gets locked up with these things and does not attack the pest or disease organisms that are the target.

1) Thoroughly clean the tank, vacuum the gravel, scrape the walls... Remove all possible non-target organic matter.
2) Treat with PP at such a dose that it stays pink through the treatment time.