when is whitespot not whitespot

Anyone got any ideas on a possible misidentification of whitespot here
- it's certainly confusing me - sorry for the long background, but
it's needed.

Background

On one of my tanks (120 litre) which is relatively peaceful my bronze
cory developed a distinct white spot just above an eye two weeks ago,
I started to treat with Waterlife Protozin and in fact thought I might
lose the cory, as it effectively hid and was inactive for a few days.
Since the tank has clown loaches in I halved the dosages as
recommended but what I forgot was that the juwel has a carbon filter
in it, so the medicine probably didn't work properly, as the cory
still had the white spot the following week at the end of the
treatment

I then removed the carbon and repeated the treatment, cory still has
whitespot,

This week I've setup a 30 litre isolation tank and put in full dosage
of the medicine for the cory, I also added a mollie which had got a
bit beaten up last week and a platy, which looked like it had a small
indistinct white spot on its tail last night, in the main tank I'm
repeated the treatment for the third time at half dosage, just to make
sure the water is clean and I am gradually increasing the temperature
and adding some aquarium salt.

HOWEVER, even thought the molly and platy in the isolation tank are
starting to look a lot better, the blasted cory still has a
distinctive white spot above his eye, which is leading me to believe
that he may not have white spot but something else. He is a lot more
lively that he was 10 days ago when this fist started.

Other factors - the tanks is overfiltered (I added a fluval 3 plus) to
the existing juwel filter but have got the flow bouncing again each
other at the back of the tank,so while there's watermovement its not
over the top, there are some plants and my two ottos and four small
clowns all seem pretty happy (I'm also pretty good at water changes)


Question

Does anyone have any ideas, I'm off on holiday in two weeks and I
would like to leave the fish in good condition.


Cheers Brian