NetMax wrote:

A proper list would be a very good thing to have. Kudos for such a good
question. Since the motivation is to know what fish are sensitive to
certain medicinal concentrations, the list should have 3 parts to it,
scaleless fish (ie: botia and some other loaches like Kuhlis),
incompletely scaled fish (ie: plecos, corydoras have the same sensitivity
due to their underbelly scaleless skin), and scaled fish (this is an
easier category : any fish smaller than 1 inch would be vulnerable).

Uhm, I don't want to correct you here Netmax, but perhaps I should...

Loaches aren't scaleless. They do however have near-microscopic scales.
Catfish on the other hand are scaleless. AFAIK all of them. They may
have armor plating, but the plates are not scales.

They are however all considered sensitive to medications, although the
exact reason for this is unknown to me. Especially malachite green is
nasty in that it seems hard to predict which species will be sensitive
to it, and which wont. However, is malachite green still available in
the US? I believe it's been outlawed here in the EU (or at least is in
the process of being outlawed).