Difference between modern fish sauce and Roman fish sauce, or difference between low and high quality garum?

I don’t know the methods of making modern fermented fish sauce, but the big difference with garum quality was location/fish. I forget which kind exactly, but there was a particular fish off the coast of modern-day Portugal which was extremely highly valued as an ingredient for expensive garum, whereas cheap garum would often use a lot of small (and different species of) fish mashed in together. “We made this out of whatever we had, except for the best catches; those went to make good garum” sort of thing.