nope. tried everything. the residue is calcium carbonate caused by mixing too much salt at one go.
because i dumped one whole pail of salt into the water, momentarily the pH and calcium levels will increase dramatically and not slow enough for it to diffuse and reach saturated levels.
this caused the calcium to precipitate out of solution. like how it will happen when you dose too much calcium and buffer at once. it's called the "snowstorm" effect.
calcium carbonate is very poorly soluble in water (if it's soluble at all...). the only way to remove it is to do so manually as it won't go away by itself.
so i've emptied my sump, scrubbed it clean, and remove all traces of calcium carbonate from there. now the problem is the main tank lol. it's still a winter wonderland.
well, it's a very tedious and expensive lesson to be learnt cost me 1 oceanic bucket..