Both the fish species you mention, are soft, to neutral water species (pH at or below 7), originally from the mineral free, acidic waters of S America.
If you have have hard, high pH water (anything above 7).... this could be the problem, and .......escpecially if you combine those parameters with a nitrate concentration above 5 ppm.
That is a chemical soup that soft water species, often have a hard time with.
Most natural waters contain undetectable nitrate. (even the hard water where I collect cichlids)
Soft water, low pH species have evolved to live with little resistance to bacterial species such at those that cause chronic diseases, like HITH bacteria that thrive in hard, high pH waters.
And adding more metallic compounds to that soup may even exacerbate the problem more.
These chronic maladies (like HITH) do not develop over night, so curing them is not instantaneous either, there is no magic bullet.
To keep infections from returning....frequent, large water changes that reduce nitrate to very low levels could help.
I try to do enough water changes, and use planted sumps that consume nitrate, enough, to keep my tanks nitrate levels, similar to the water I collect the cichlids in.

