How do I lower the salinity in my reef tank?
Take saltwater out and put fresh water in. Salt only leaves the system in a bucket, so the question is how much to swap: a 100 gallon system at 37.5 ppt coming down to 35 needs about 6.7 gallons of tank water replaced with RO/DI (25 L) — that's 6.7% of the system, because dilution scales as one minus the ratio of the two salinities. If your sump has the room, the same job is adding 7.4 gallons of RO without removing anything (the level rises; your ATO will simply not top off until it evaporates back down). Do it over 3 days at 0.83 ppt a day — about 2.2 gallons swapped per day — not in one go.
Two ideas that sound right and aren't. When water evaporates the salt stays behind, so evaporation only ever raises salinity — "let it evaporate for a while" moves you the wrong way. And an auto-top-off filled with RO holds the level, not the salinity: it replaces exactly the pure water that left, so a system that reads 1.028 with a working ATO will read 1.028 tomorrow. To come down you have to remove salt, which means removing saltwater — swap it for RO, or do the water changes with lower-salinity water. The one thing an ATO does help with: it lets you top up with RO after a swap without thinking about the level.
How do I raise the salinity?
Add salt as dissolved saltwater, never as dry salt into the display. The same 100 gallon system at 32.5 coming up to 35 needs about 1,061 g of a typical reef mix (1,011 g of pure salt; the calculator scales for your brand's moisture) — dissolve it in a bucket of tank water, or mix a small batch of very salty change water, and add it slowly to the sump or a high-flow area over 3 days. The other route, which many people prefer because it needs no weighing: make the next water changes saltier than the tank. Mixing change water at 40 ppt and doing 10% moves a 33 ppt tank to 33.7; to reach 35 in one change you'd need to swap 29% of the system, which is why the calculator's "via water changes" mode also solves the fraction, and why a slow climb over several changes is the pleasant way.
How fast can I change salinity?
About 1 ppt a day in an established reef, and slower coming down when there are shrimp, snails and other invertebrates in the tank — they osmoregulate poorly and mind a drop far more than a fish does. Big swings are how new keepers lose their cleanup crew while "fixing" a number that was only wrong by two points. Coming up is gentler on livestock but easier to overshoot, because concentrated salt water added fast makes a local spike; the pacing box in the calculator applies either way. Two caveats: hyposalinity treatment for fish is a deliberate, different procedure with its own schedule (this tool isn't it), and none of this is worth doing until you've confirmed the reading — a refractometer zeroed on RO water reads a point or two high, which the salinity converter explains before you pour anything.
A 100 gallon system at 37.5 ppt holds about 14.7 kg of dissolved salt; at 35 it should hold 13.7 kg. Removing 6.7 gallons of tank water takes 0.98 kg of salt out with it, and the RO you put back brings none — and that's the whole correction. Salt-mass bookkeeping is the entire calculation; the temperature and density terms the engine carries along matter at the second decimal.
Doing the swap without a mess
Turn the ATO off first, or it will replace what you siphon before you can. Siphon the day's share from the sump — the return chamber, so you pull mixed system water and not detritus — into a bucket, then pour the same volume of RO/DI back into a high-flow chamber slowly, over a few minutes, so nothing downstream sees a slug of fresh water. Turn the ATO back on. Retest the next day at the same time with the same instrument, and expect a slightly smaller move than the calculator promised: the true system volume is a guess for most people (the true volume tool tightens it), and rock and sand hold pockets of the old water. Two or three slow rounds land it; one big correction "to be done with it" is how snails end up on their backs.
Which number should I aim for?
Natural seawater is 35 ppt (SG 1.0264 on a calibrated refractometer) and the reef consensus is 34–36. If you're inside that band, the honest advice is usually to hold what you have and make sure the change water matches it — the salt mix calculator takes the tank's salinity as a target for exactly that reason. Correct on purpose, once, slowly; then let stability do the work.