>3) Ni2+ can increase background. Avoid if possible.
Another problem of using Ni2+ is fading. Ni-containing Black DAB
staining tends to fades faster than brown staining without Ni.
After five years, Black staining may have gone away (depending
on the mounting procedure). And sometimes you DO want to
re-examine your preps five years later.
Also, if you want to post-fix (after DAB reaction) the specimen
with glutaraldehyde, avoid using Ni2+. The staining may go away
quickly during this procedure.