I actually experimented with malachite green when I lived in the UK, as I noticed it was very popular over there. I used it (for 30 days) to treat fish with Freshwater Ich, Crypto and Amyloodinium. While the fishes' condition did improve somewhat during treatment, the white spots kept coming back throughout which proved to me either: a) MG is not capable of eradicating ALL theronts/dinospores b) I wasn't using a strong enough concentration of MG (but I wasn't too far from the lethal dose). Most of the fish with velvet eventually died, but I did save all the fish with FW & Marine Ich by treating for 30 days with Cupramine. I used a colorimeter 2x daily to measure malachite green concentration in the water. So, this is why I am so confident that it's not 100% effective against ectoparasites. Now, why your LFS thinks using malachite green in combination with Cupramine makes the latter more effective at a lower concentration ... I cannot say and quite frankly, it doesn't make any sense to me.
Now, I am very interested in malachite green (and methylene blue) capabilities as a antimicrobial, as I recently had success using it to treat bacterial infections in freshwater fish (my elderly father likes to buy fish from Walmart
). I plan on experimenting with it again soon for that purpose only. But not as an antiparasitic; however using it in conjunction with copper might prove useful if the fish also has a bacterial infection.With regards to the temperature thing, there is no direct scientific evidence that warmer water speeds up Crypto or Amyloodinium's lifecycle. However, it has been proven correct for FW Ich (Ichthyophthirius multifiliis) so it is at least plausible for Marine Ich as well. But I wouldn't count on it until that theory has been confirmed. To my knowledge, the only experiment that has been done with regards to Crypto & temperature is that 40C for 1 hour can be used for disinfection.
