One of the infuriating things about the Romans is that they didn’t do documentation, or if they did, an early medieval monk didn’t find it titillating enough, and used it to light the fire or worse. In very crude terms that’s why we know rude stories about Theodora and geese, and actually don’t know about fish sauce.
Yes we do know the Romans liked fish sauce, that they called it garum, or sometimes liquamen, and that sometimes over the thousand years of fish sauce consumption they seemed to be synonyms and sometimes they referred to different things. We don’t really understand the distinction, or why it changed, or indeed exactly what went into each.
Just like in Laos. What in English we call ‘fish sauce’ can refer to a number of possibilities. Basically, there’s two sorts of fish sauce, a thin brown one that’s identical with nam pla, which you can buy in any Vietnamese or Thai supermarket, and Padeak, a thick gloppy fish sauce made of fermented pickled fish and which often has chunks of fish in it. Not surprisingly, across the border in the Lanna lands of north east Thailand they make something similar called pla ra. Pla ra is smelly but loved. It’s also been used in political protests to make stink bombs to throw at politicians, but that’s a different story. As far as I know, the Romans did not make a habit of throwing garum at opposition politicians.
My guess, and it is only a guess, is that liquamen was like nam pla. Yes they probably tasted a bit different and were used differently, but basically a thin sauce added for flavour during cooking.
Garum was probably more like pla ra, a spicy addition added to the meal to give you that spicy fish taste with that extra protein from the chunks of pickled fish.
Or of course I could be totally wrong – if you want a different take check out ‘Pass the Garum‘ …