Water chemistry, demystified
Salinity, alkalinity, calcium, magnesium, nitrate, phosphate — what each parameter does, what range it belongs in, and how to test accurately without a lab.
Parameters first. Livestock second.
Most reef tanks crash in the first six months. Here's why.
Reef Directory covers saltwater aquarium setup, water chemistry, and livestock selection without the forum mythology — just the parameters and decisions that actually keep coral and fish alive.
8.1–8.4target pH range for a stable reef
420–450alkalinity in mg/L (dKH 8.3–9.0) corals need to calcify
6–8 weeksminimum cycle time before adding livestock
What we cover
The fundamentals most guides skip
The nitrogen cycle
Every tank crash traces back to skipping or rushing the cycle. We cover ammonia, nitrite, and nitrobacter colonization — and how to confirm the cycle is actually complete.
Equipment: what matters vs. what is sold on hype
Skimmers, return pumps, lighting spectra, reactors — which pieces of hardware move the needle and which are hobbyist folklore.
Livestock compatibility and order of introduction
Fish that eat coral, corals that sting each other, and the specific sequence in which to add animals to a new system — all of it before you're at the fish store.
Coral husbandry by category
SPS, LPS, and soft corals each have different flow, light, and chemistry requirements. Know the tier you are actually ready for before you buy.
Troubleshooting without guessing
Bleaching, RTN, cyano, dinos, flatworm blooms — what causes each crash, what to test first, and what interventions have evidence behind them.
The short version
How to start a reef tank that survives
01
Cycle the tank completely
Run ammonia through zero before adding any livestock. Six to eight weeks minimum. Test daily through the nitrite spike. There is no shortcut that does not cost livestock.
02
Stabilize water chemistry before coral
Hold salinity at 1.025–1.026, alkalinity at 8.3–9.0 dKH, calcium at 400–450 ppm, and magnesium at 1250–1350 ppm for two weeks straight. Only then add your first frag.
03
Add livestock slowly and in the right sequence
Cleanup crew first, then a single hardy fish, then coral frags — weeks apart. Each addition raises bioload and tests your parameters. Patience here prevents the crashes that define the hobby's first year.
Questions from new reefers
How long does it take to cycle a new reef tank?
Six to eight weeks for a tank with no shortcuts. Add an ammonia source (pure ammonia or a raw shrimp), then test daily. The cycle is complete when ammonia and nitrite both read zero after you dose ammonia — not before. Bacteria seeding products can shorten the curve to three to four weeks in a best case, but you still need to verify with tests, not a calendar.
What is the most important water parameter to keep stable?
Alkalinity. It is consumed by coral calcification daily, fluctuates faster than calcium or magnesium, and wild swings cause bleaching and RTN faster than almost any other parameter error. Test alkalinity every one to two days when you have SPS coral, and dose to keep it within 0.5 dKH of your target.
Can I keep coral under LED lighting?
Yes. Modern aquarium-specific LEDs — Radion, Hydra, Kessil — deliver the 420 nm and 450 nm spectra coral zooxanthellae use for photosynthesis. What matters is PAR (photosynthetically active radiation) at the coral's position in the tank, not the fixture brand. Target 50–150 PAR for soft coral and LPS, 150–350 PAR for SPS.
Do I need a protein skimmer?
For most reef tanks, yes. A skimmer removes dissolved organic compounds before they break down into nitrate and phosphate. At low bioload and high water-change frequency you can run skimmerless, but most hobbyists find it harder to keep nutrients in range without one. Oversize the rated capacity by 30–50%.
What fish are safe in a reef tank?
Generally safe: clownfish, gobies, dartfish, basslets, cardinalfish, and most tangs. Risky: wrasses (some eat inverts), triggers (eat inverts and small fish), puffers (eat coral and inverts), lionfish (eat smaller fish). Individual variation exists — research the specific species, not just the family. Add fish after corals are established so territorial disputes favor the coral.
Insights
From the blog
Water Chemistry
Saltwater Aquarium Water Parameters: The Numbers That Keep Coral Alive
Complete reference for reef tank water parameters — target ranges, testing frequency, and what happens when each value drifts outside its window.
SetupReef Tank Setup: The Sequence That Actually Works
A step-by-step reef tank setup guide covering equipment, cycling, and the order to add livestock — with timelines that reflect how long the biology actually takes.
Get the parameters right before you buy anything
Water chemistry is the whole game. Start with the numbers that matter and what range to hold them in.
Water parameters guide