A new study has shown that most modern treatments for whitespot are not as effective as the old-fashioned ones.
For many years, the triphenylmethane dye malachite green has been used (often in combination with liquid formaldehyde) to treat fish infected by the whitespot parasite, Ichthyophthirius multifiliis.
However, several studies have shown it to be a carcinogen (cancer-causing agent), and many countries have banned its use, or are planning to ban its use in certain applications in the near future.
Several alternative medications for whitespot have been developed, but there has been some debate as to how effective they are compared to the widely used malachite and formalin combo.
Now scientists at the University of Oulu in Finland have shown that, with the exception of one treatment, all are less reliable than the malachite green and formalin products they are supposed to replace.
The study tested the performance of formalin, potassium permanganate (KMnO4), chloramine-T, hydrogen peroxide and two new chemicals called Per Aqua and Desirox.
The latter two products, which are mixtures of acetic acid, peracetic acid and hydrogen peroxide, were also tested on their own, and in conjunction with formalin, which isn't yet banned.
The results showed that all of the chemicals were able to successfully lower the parasite burden so that the mortality rate dropped within a month of the fish picking up an infection.
However, with the exception of one product, none were as good as the malachite formalin mix:
"Large differences in parasite burden and mortality occurred among the replicates in all except the Desirox-formalin tanks, which means that they are not as reliable as the malachite green formalin used previously.
"It was also evident that the chemicals and their concentrations must be planned carefully to suit the conditions of each farm."
For more information see the paper: Rintamki-Kinnunen P, Rahkonen M, Mannermaa-Kernen AL, Suomalainen LR, Mykr H, Valtonen ET (2005) - Treatment of ichthyophthiriasis after malachite green. I. Concrete tanks at salmonid farms. Dis Aquat Organ. 2005 Apr 6; 64(1): 69-76