To treat or not to treat (tank with latent infestation?)

Deviating a little from the standard template because I don't have just a single ill fish or tank...

I have a main tank. I also have several small tanks. I keep a larger community in my main tank, then sometimes move out pairs or singular fish to breed with. The tank has been established for several months and contains fish from several different shops. All tank inhabitant (except the first) were quarantined for at least 3 weeks prior to being added to the tank. Of these, the only one that ever showed illness and still ended up in the tank (I did have a few QT deaths) was a gourami with fin rot to her caudal. Healed up nicely, left her in the QT for a month.

Anyway -- fast forward to today. I appear to have an issue with several of the breeding fish, that get something that is either ich or velvet (haven't really been able to identify it, it has the 'dusty' texture and size of velvet but the snowy white colour of ich with not a hint of gold) when they are moved into the smaller tanks and exposed to the naturally more stressful and tiring process of breeding. Some of these tanks were brand new set-ups and could not possibly have contained the parasite beforehand. Hence, whichever parasite it is must be present in my main tank. I've also seen the very occasional flashing in the main tank but kind of learned to pay it no heed as the fish never fell ill, it happened rarely and I never saw any spots or flecks appear. I've heard healthy adult fish can sometimes harbour the parasite and even become (half) immune, so they have the parasite living in them but it won't really make them sick until their immune system is lowered, and main tank is a good enough environment that that doesn't happen.

Which leads me to -- what now?
I've been thinking about just dosing my entire main tank (and all breeding tanks, and QT tanks) with esha exit. But the main tank also includes Dojo loaches, and one of the other tanks that houses a breeding fish that I think has been exposed houses ADFs as well. I'm extremely hesitant to use anything that risks the health of either the dojos or the ADFs as I've had both for a long time and am attached to them...
Is it at all an option to just move all the breeding stock to a new community, treat that one and then just leave the latent parasite in my loach tank? Or do you guys think it would be better to preventatively treat them as well? I know it has been claimed that esha products are quite safe with otherwise sensitive fish but I'd rather not take the gamble. Does anyone have any actual experience (or direct me to someone who has) using exit with scaleless loaches?

(Note: heat, salt or darkness treatments are really not an option as the tank is very heavily planted).

As for the infection itself... how long can a fish live with velvet before it becomes really detrimental? It looks more like velvet than ich but I failed to recognise it as something other than iridesence in the beginning because of my QT practices and just though it looked like dusting because of the tank light differences.. poor fish had for a month or so before I treated it, and only towards the end of the month started clamping fins etc. It does wax and wane in how much dusting there appears to be so it does seem to be a skin parasite... yet it seems most of what I hear of velvet is that is quite deadly and kills really fast, so maybe a weird strain of ich after all?
Picture of the fish in question below.

Hi and welcome to the forum

I don't think it's velvet or whitespot, however I need a better picture and if possible a short video of the fish rubbing to be sure.

My guess is either poor water quality, chemicals in the water (possible overdosing of plant fertiliser), or an external protozoan infection like Costia, Chilodonella or Trichodina. With an external protozoan infection being most likely if the fish has cream patches on the body as shown in the picture.

Fish can live with Costia, Chilodnella and Trichodina for months and the problem only gets worse if you miss a water change or the parasites are allowed to build up in numbers.

If it is an external protozoan infection (whitespot and velvet are also external protozoan infection), you can treat it with Malachite Green or Copper. My preference is copper but it will kill invertebrates like shrimp so they need to be moved out, as will the frogs.
Malachite Green is carcinogenic (causes cancer) but also treats external protozoan infections. However, it is not safe for frogs.

The frogs should be housed in their own tank because they are not suited to community tanks. They eat small fish and shrimp in the tank and if you need to treat the tank, the medications used on the fish normally kill the frogs. Frogs should always be kept in their own enclosure and not mixed with fish.

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You need to check the water quality for ammonia, nitrite, nitrate, pH and GH and post the results here. If these are ok and there is 0 ammonia, 0 nitrite, and less than 20ppm nitrate, and the pH and GH are suitable for the fish, then you can look at chemical medications. But test the water first and post more pictures so we can make a more accurate diagnosis.

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How often do you do water changes and how much do you change?
Do you gravel clean the tank when you do a water change?
Do you dechlorinate the new water before adding it to the tank?

What sort of filter do you have?
How often and how do you clean the filter?