The fish, a CICHLID, was shot on colour slide in a local fish
dealers after asking permission. I also let them have a few
prints.

The tanks were beautifully clean in every sense, with glass clean
on the inside and out.

I use flash off the camera and hold, or set, at the flash at
about 45 degrees to the glass, pointing at the subject, with the
camera lens PARALLEL to the glass. In this way, any reflected
light does not go into the camera lens.

This method works for almost anything behind glass.

Exposure is another matter. 50% of the light can be lost by
reflection, so calculated exposures can be difficult. Water will
also absorb a lot of light.

Modern auto flash systems can help, but can also let you down
badly. The only real answer is to try different exposures about
an average that experience tells you is about right.

For mono and colour print I always over expose a bit on the
assumption that an over exposed negative gives better results
than one underexposed.

For colour slides, the exposure must be correct.