while this is only my personal guess & theory.
add snails, ... not because you want a snail problem, ... but the snails will grow, their shells will be full of calcium, and remove the excess snail (taking with it all that calcium - your white residue that's all over the place)
i have been thinking of ways to remove excess nutrients. so far all i can think of calcium (naturally without some special chemical reactor)
other excess nutrients i'm at a loss about removing other than through plant trimmings.
reef tanks use a reactor that can remove phosphates (iron i think is what that reactor uses)
-i guess in high pH tanks this isn't an issue as higher pH levels limit iron availability.
phosphates can be removed through their bond to calcium as well
hmmm, not bad