Poorly Phantom

Looking at the diseases in the link -
Unless you are feeding some very cheap unbranded food, you can probably rule out lack of vitamins.
Fish TB has other symptoms you haven't reported, and is quite rare in freshwater aquariums - though not unknown.
Osmoregulation problems are usually when you keep fish that like very hard water in very soft water. It is not common the other way round. Black phantoms are not hard water fish.

That leaves the three parasitic infections. I have not heard of Gyrodactylus before, though I have heard of costia and velvet.
I would think that Gyrodactylus is unlikely as the link says it gets into aquariums "with feed taken from ponds and not quarantined for 10 days" - unless you've been doing that.

Flagellates are protozoa aren't they? Both costia and velvet are caused by flagellates. If they are protozoan, eSHa 2000 should get rid of them, according to their website.




But......

Since the problems occur after a water change, could it be your water conditioner? I've just googled it and found people making comments about it leaving white swirly things in the water thought to be caused by the aloe vera it contains.
You are using the correct dose of the conditioner? Hagen's website says 5ml per 2 litres.

In your circumstances, I would try a different water conditioner. If you want to try something that contains only thiosuphate (to remove chlorine) and EDTA (to remove metals) you could look at API Tap Water Conditioner. That's the one I use, and it doesn't have anything to 'stimulate the slime coat' etc. But it is quite hard to find in shops, I often have to buy it on line if we don't go to the not very local shop that sells it.

The only other thing that comes to my mind is the water itself, since the problems start after a water change. Have the suspect water changes been on the same day of the week? I ask because a local shop once told me that several of his customers had fish die after a water change, all done on Saturdays. One of the victims contacted the water company and they said they did something on Saturdays. Depending on how garbled it was when it got to me it was either flushing something through the pipes to prevent limescale build up or to remove limescale build up. Does your water company do something similar?
American water companies are known to add extra chloramine after exceptionally heavy rainfall so fishkeepers then have to use extra dechlorinator.




  Basically, I don't know what the problem is, I'm grasping at straws