1. Evaporation concentrates salt
When fresh water evaporates from a freshwater tank, nothing else changes. In a reef tank, every gallon that evaporates leaves its salt behind and raises salinity in what remains. Losing 2 gallons from a 50-gallon system pushes specific gravity from 1.025 to about 1.026. Knowing the real system volume tells you how much top-off water to add and how fast salinity drifts.
2. Specific gravity changes weight
Reef water at specific gravity 1.025 weighs 2.5 percent more than fresh water: 8.55 pounds per gallon instead of 8.34. On a 180-gallon system that is an extra 38 pounds. Every calculator on this site shows both figures.
3. Total system volume matters more
Reef systems usually include a sump and often a refugium, and all of that water shares the same chemistry. Alkalinity, calcium and magnesium dosing is calculated on the whole system. A 90-gallon display with a 30-gallon sump is a 115-gallon system after rock and sand. Use the sump calculator to add it up.
4. Live rock and sand displace a lot
A typical reef carries one to two pounds of rock per gallon and a one- to two-inch sand bed. On a 75-gallon tank that can be 6 to 10 gallons of displacement, far more than the driftwood and gravel in a freshwater tank. The displacement calculator handles rock by weight and sand by depth.
At a glance
| Factor | Freshwater | Saltwater / reef |
|---|---|---|
| Weight per gallon | 8.34 lb | 8.55 lb at SG 1.025 |
| Evaporation effect | Level drops | Level drops and salinity rises |
| Volume used for dosing | Display tank | Display + sump + refugium |
| Typical displacement | 2–5% | 8–15% |