How to Lower Nitrate in a Discus Tank

If you want to lower nitrate in a discus tank, start with your tap water, not your filter. Many UK water supplies can contain 30–40ppm of nitrate before a single fish is added, and the level often changes through the year. My water supplier’s report lists 40ppm, and for a long time my discus tank sat at 40ppm or more. Here’s what finally brought it right down.

Why nitrate matters for discus

Nitrate is the end product of the nitrogen cycle: fish waste and uneaten food become ammonia, then nitrite, then nitrate. It’s far less toxic than ammonia or nitrite, but discus are heavy eaters kept warm, so it builds fast. Long-term high nitrate is linked to stress, poor colour, slow growth and a higher chance of health problems like hole in the head.

Target: keep nitrate under 20ppm, and ideally around 10ppm. See discus water parameters for the full set of targets.

Step 1: Test your tap water first

Test the water straight from the tap, and check your water supplier’s annual quality report too (search your postcode on their website). Don’t rely on a single test: tap nitrate can vary a lot through the year, and mine sometimes tests low even though the report shows 40ppm. Test every few weeks and plan for the higher figure. Whatever your tap water contains is your floor, because a water change can only bring the tank down towards that level, never below it.

Tap water nitrate changes all the time. Water companies blend supplies from different sources, such as boreholes, rivers and reservoirs, and switch between them depending on demand and the season. Rainfall and farm run-off also push levels up and down. So your tap water might test at 5ppm one week and 40ppm the next. That’s why it’s worth testing new water before every big change, and why a steady routine like an RO mix protects your discus whatever comes out of the tap.

Step 2: Dilute with RO water

Reverse osmosis (RO) water has almost no nitrate. Mixing it with tap water cuts the nitrate in every water change. I now use a 50/50 RO and tap mix, which halves the nitrate going in while the tap water keeps enough KH to hold the pH steady.

Does RO water have zero nitrate? Not quite. An RO membrane typically removes around 90–98% of nitrate, so 40ppm tap water might come out at 1–4ppm, and more as the membrane ages. If you want true zero, add a DI (deionisation) resin stage after the membrane. A cheap TDS meter on the RO output is worth having too: when the reading starts creeping up, it’s time to replace the membrane or pre-filters. For a 50/50 mix, a few ppm from the RO side makes no real difference.

Don’t use straight RO without adding minerals back – it has no buffering, and the pH can crash. I tried remineralised RO and found most discus mineral products gave almost no KH, so the pH kept dropping. A mix with tap water has been far more stable. More on this in RO water for discus.

Step 3: Big, regular water changes

Water changes are still the main way nitrate leaves the tank. On my 540-litre tank I change 100 litres twice a week, vacuuming the sand at the same time to remove waste before it breaks down. Matching the temperature makes big changes safe – see easy water changes for discus tanks for how I do it without buckets.

Step 4: Let plants do the work

Fast-growing plants take up nitrate as food. A well-planted tank, or a top sump planted with fast growers above the filter, pulls nitrate out between water changes. Once my plants were growing properly again, nitrate and algae both dropped. Choose plants that cope with discus temperatures – see best plants for a discus tank.

Step 5: Nitrate removal resin

Resins can be used to change particular water parameters in an aquarium. Here the output of the filter is passed through nitrate removal resin in an additional filter booster canister. This makes swapping the resin for recharging quick and simple, without disturbing the main filters. With the full flow forced through all of the resin, it works far more efficiently and nitrate drops very quickly.

Step 6: Cut nitrate at the source

Feed what gets eaten. Remove leftovers after a few minutes – uneaten beef heart rots quickly in warm water.
Keep the substrate clean. Vacuum sand at every water change so waste can’t build up.
Don’t overstock. More fish means more waste – see best tank size for discus.
Clean the filter sponges. Trapped muck keeps producing nitrate – rinse mechanical media in old tank water.