Nitrate is the end product of the nitrogen cycle. It is much less toxic than ammonia or nitrite, but it builds up steadily. High nitrate stresses fish, weakens their immune system, fuels algae and, in reef tanks, harms sensitive corals.
What level is safe?
- Community freshwater tanks: below 20–40 ppm
- Sensitive fish (discus, many wild-caught species) and shrimp: below 10–20 ppm
- Planted tanks: 5–20 ppm; plants actually need some nitrate
- Reef tanks: roughly 1–10 ppm depending on the corals; near-zero nitrate can make corals pale
Why nitrate rises
- Not enough water changes for the fish load
- Overfeeding
- A dirty substrate or filter full of trapped waste
- Too many fish for the volume
- Tap water that already contains nitrate. Test your tap water too.
How to lower it
- Water changes are the fastest fix. A 50% change roughly halves nitrate. Above about 50% at once, split it over 2–3 days to avoid shocking fish with sudden changes.
- Feed less. Give only what is eaten within two minutes, and skip a day each week.
- Vacuum the substrate and clean the mechanical filter media in tank water.
- Add fast-growing plants. Floating plants and stem plants such as hornwort take up nitrate directly.
- Check your tap water. If it already has 20+ ppm nitrate, use RO water or a nitrate-removing resin.
- Reef tanks: keep the protein skimmer efficient, and consider carbon dosing or a refugium with macroalgae.
Keeping it low
The goal is balance: the nitrate your tank produces each week should be removed each week. If nitrate keeps climbing even with your normal routine, either increase water changes, reduce feeding or stock, or add more plants.
When nitrate is too low
In planted and reef tanks, nitrate at zero can also be a problem. Plants yellow from the oldest leaves, and corals lose color. Then dose a nitrogen fertilizer (plants) or feed more (reef) until it is back in range.
Fishtify calculates the right nitrate range for the fish, plants and corals you keep and tells you how big a water change you need.