Freshwater chemistry is mostly about what you don't want in the water. Reef chemistry is about what you do want — in precise, stable amounts.
When people say reef keeping is harder than freshwater, this is most of what they mean. Corals are living animals that build their skeletons from elements dissolved in seawater. If those elements aren't present in the right concentrations, the coral either can't grow, loses color, or dies. You're not just maintaining water quality — you're maintaining a miniature ocean.
Here are the parameters that matter, in rough order of how quickly problems show up when they're wrong.
Salinity
Natural seawater runs at about 1.025–1.026 specific gravity (35 ppt). That's your target for a reef tank. Fish-only systems can tolerate a slightly lower salinity which can help with disease management, but corals and invertebrates need natural seawater levels.
The main risk is evaporation. As water evaporates, salt stays behind and salinity creeps up. A consistent top-off system — either manual or automated — is essential. Even a few days of neglect can push salinity out of range in smaller tanks.
Get a refractometer, not a swing-arm hydrometer. The plastic hydrometers are inaccurate and get worse over time. A decent refractometer is $30 and will serve you for years.
Alkalinity (dKH)
Alkalinity — usually measured in dKH (degrees of carbonate hardness) — is the parameter that catches new reef keepers off guard. Target range is 8–12 dKH, with most experienced reefers picking a number in that range and keeping it stable rather than chasing the high end.
Corals consume alkalinity constantly as they build their skeletons. In a lightly stocked system with a few soft corals you might only need to dose occasionally. A heavily stocked SPS system can consume alkalinity fast enough that you need to dose daily or run a dosing pump. Test alkalinity at least twice a week until you understand your tank's consumption rate.
Alkalinity swings are more damaging than a stable low value. A tank running consistently at 7 dKH is better than one swinging between 7 and 11. Stability matters.
Calcium
Target range is 380–450 ppm. Calcium and alkalinity are linked — corals use both to build calcium carbonate skeletons, so a tank with growing corals will consume both. They also interact chemically: pushing one too high can cause the other to precipitate out of solution.
If your alkalinity is in range and your calcium isn't, or vice versa, the usual culprit is an imbalanced two-part dosing solution or a poorly calibrated calcium reactor. When in doubt, test both on the same day.
Magnesium
Target range is 1250–1350 ppm. Magnesium doesn't get consumed as quickly as calcium and alkalinity, but low magnesium makes it hard to keep calcium and alkalinity stable — they'll keep precipitating out no matter how much you dose. If you're fighting to maintain your parameters and nothing is working, test magnesium. It's often the hidden culprit.
pH
Reef tanks target 8.1–8.3. pH in a closed system fluctuates throughout the day — lower at night when corals and algae consume oxygen and produce CO2, higher during the day when photosynthesis drives CO2 down. A swing of 0.2–0.3 units across the day is normal. More than that suggests poor gas exchange or a CO2 problem.
Running a refugium with a reverse photoperiod (lights on at night when the display tank lights are off) smooths out pH swings naturally. Increasing surface agitation improves gas exchange. Some reefers run a calcium reactor which acidifies the water and depresses pH — this needs to be compensated for.
Nitrate and phosphate
Unlike freshwater where the goal is just "as low as possible," reef tanks need some nitrate and phosphate or corals will starve. The old zero-nutrient approach caused a lot of bleached, unhealthy corals before the hobby figured this out. Modern targets are nitrate 1–10 ppm and phosphate 0.03–0.10 ppm. SPS corals generally prefer the lower end of those ranges; LPS and softies are more tolerant.
What to test and when
Logging these consistently in Tanqly lets you see trends over weeks rather than just snapshots. A slowly declining alkalinity trend is much easier to catch on a chart than in your memory of the last few tests.