Malachite - therapeutic dose

[Locking Nut]

Hi all. First post - but 25 year koi keeper so please go easy! Something is confusing me a bit and would appreciate some pointers:

Background is that I bought eight new fish a few months ago, all currently held in a completely self-contained quarantine pond.

Within a couple of weeks there was sign of spirolagnia on the flanks of one of the newbies, a kujaku - and on the pectoral fins of another, an aka matsuba. Slightly frustratingly, close examination of the photos I took of the fish the day they arrived showed that the matsuba was already exhibiting signs of the infection - which on initial check looked like natural fin colouration. Anyway...

I'd historically treat this this with Blagdon anti-fungus & bacteria (acriflavine and malachite) but on this occasion came home with NT Labs Malachite in part because I hoped it'd have less of an effect on the filter. I turned off the UV, removed the wool pack from the filter, added the recommended dosage and watched - and sure enough the fungus reduced on both fish but was not eliminated. I tried a second course a couple of weeks/partial water changes later, and again reduction but not eliminated. Then tried adding salt to 0.5% and again reduction in visible symptoms but not eliminated. Now I've started to change out and reduce the salt level it's spreading again - on the kujaku in particular - and I need to get on top of it somehow before the water gets too cold.

One of the things that's notable is that the Blagdon treatment is one dose per day for five days. The NT Labs is recommended a single dose and then leave for a week. What was interesting is that the NT product only managed to dye the water visibly for about an hour - even with the UV off and filter wool out, whereas the Blagdon, whenever I've used it in the main pond, will maintain a significant amount of water discoloration from one day/dose to the next.

So I'm wondering whether the filter on the quarantine pond it is removing the malachite before it can effectively treat the fish (and therefore I need to repeat the dose, or possibly apply it directly to the fish) - but equally don't want to overdose. Does Malachite need to be visibly dying the water to be effective?

The other fish appear completely unaffected by the lurgy - it's just these two, so I'm already considering setting up a separate quarantine (possibly indoors) for the pair of them and medicating them in that, or trying the Blagdon product that's worked for me in the past - any advice/pointers much appreciated!