I would like to change the loottable of neptunes bounty. But apparently Aquaculture 1.12.2 doesn't have a single loottable so that is not possible.

Well I mean this wasn't really a "yeah this is how your mod works" more like a request of adding that to the 1.12.2 version if that wouldn't be too much work.

Correct. Which is why the whole system got scrapped in Aquaculture 2 (For 1.13+) and moved to loot tables to allow for more customizing by the users of the mod.

And I'm saying that the whole mod had to be rewritten to allow for that, hence Aquaculture 2.