You are well out of my expertise if you want to go deeper into tds, it stands for 'total disolved solids' and therefore is what is disolved in the water as regards, metals, minerals etc. Why different types of shrimp need different quantities is beyond my knowledge, I just accept the fact! This is why I buy specific shrimp products like gh/kh+ as I let the experts get the balances correct.
Obviously tds is easier to adjust as you are dealing with a different scale than gh, but by increasing the tds it will also increase the gh, but maybe not enough to show up on a gh test as it may be only say from 3.1 to 3.3 but the test kits only show whole numbers (so both would show gh 3 on a gh test kit), that is why it is more sensible to just use tds at this point, and that is easier to test anyway if you have a tds pen/meter.