[shrimpchips]

Are you using ash soda (Na2CO3) or sodium bicarbonate (NaHCO3)?

If you're using soda ash, the additional sodium could be the source of your salinity woes. With twice as much sodium being dumped into your tank, the salinity will rise that much faster.

Also, what's your pH? I suspect it's on the higher side also since you're not getting the H+ from your Alk supplement (among other things). I know the amounts may seem trivial, but I think the combination of a high pH and high amount of CO3 and Ca you're having to add to the system may be causing your woes.

I would try switching your Alk supplement to sodium bicarbonate and see if that can change some of your fortunes. It sounds crazy that you'd need half a litre of supplement a day.....