The easiest way to keep SPS: Pick a lane.
Pick one of two ways and run all of it. ULNS, the ultra-low nutrient system, keeps nitrate at 0–1 ppm, phosphate under 0.03 ppm and KH around 6.5–8 dKH: corals color fastest, and there is no room for error. The classic way runs nitrate 5–10 ppm, phosphate 0.05–0.10 ppm and KH 8.5–11 dKH: more forgiving, slower to color. Whichever you pick, your KH has to follow your nutrients.
Enter all three and your tank lands on the map below.
Add pH, calcium, magnesium and temp
Optional. These don’t move your dot; anything out of range gets flagged.
- ULNS
- Classic
- REEF JASE
- Your tank
Chart of KH against nutrients. Nitrate and phosphate share the bottom scale, paired by lane: 0.03 phosphate with 1 ppm nitrate, 0.05 with 5, 0.10 with 10. A corridor rises from the ULNS lane (nitrate 0–1, phosphate under 0.03, KH about 6.5–8) to the classic lane (nitrate 5–10, phosphate 0.05–0.10, KH about 7.4–11). REEF JASE sits at nitrate 0–5, phosphate 0–0.03 and KH 7.5–8.0, on the border of ULNS. Past nitrate 10 the band continues greyed out: past both lanes, where corals brown out more easily. Above the corridor, KH is ahead of nutrients and SPS tips burn. Below it, nutrients are ahead of KH and corals run browner and slower. A tank plots as one point: its KH, and its nitrate and phosphate counted equally. Nitrate up with phosphate bottomed out is where corals bleach more easily.
ULNS vs the classic tank
ULNS stands for ultra-low nutrient system, the Euro/Asian way of keeping SPS. Nitrate and phosphate run so low a hobby test kit reads zero, and the corals are fed by dosing instead: aminos, bacteria, carbon and trace elements.1
It colors faster than anything else, and it gives nothing back when you slip. A KH swing, a missed dose or phosphate hitting the floor shows up in the tips fast. Even the ULNS guides say it isn’t for beginners.1
The classic tank carries real nutrients and the KH to match. Fish and food keep nitrate and phosphate on the board, the corals always have something to eat, and you can miss a day without paying for it. The catch is color: it comes slower, and corals usually go browner before they get there.
| ULNS | Classic | |
|---|---|---|
| Nitrate | 0–1 ppm | 5–10 ppm |
| Phosphate | Under 0.03 ppm | 0.05–0.10 ppm |
| KH | 6.5–8 dKH | 8.5–11 dKH |
| Runs on | Aminos, bacteria, carbon dosing, trace elements | Fish load and feeding |
| Color | Fast | Slower, browner first |
| Room for error | None | Wide |
Why do SPS corals turn brown?
Mostly nutrients. The brown is the zooxanthellae, the algae living in the coral's tissue, and nitrate and phosphate are their food. Feed the water and they multiply: in one study, adding nitrate alone pushed zooxanthellae density up and slowed skeleton growth.4
The color is the coral’s own pigment, and light controls how much of it gets made.6 When the zoox thin out, more light reaches the tissue and the coral makes more.7 That’s the whole ULNS trick: keep the zoox lean, let the light through, and the coral does the rest. It’s also why the classic lane colors slower. Those corals aren’t sick. They’re carrying more zoox.
What KH should SPS run?
Whatever your nutrients can support. Korallen-Zucht has ZEOvit tanks hold KH at 6.5–7.5 dKH,2 while a tank carrying real nitrate and phosphate can run 9 to 11.3 High KH with nothing in the water burns the tips.
KH is what a coral builds skeleton from, and the higher it runs, the faster the skeleton goes. Tissue has to keep up, and tissue is built from nutrients. Run high KH in an empty tank and the skeleton outruns the tissue: the tips go thin and burn under the light.3 So the rule runs both ways, up to a point. The more nutrients you carry, the more KH you can run, but neither lane runs past 10 ppm nitrate or 11 dKH. Drop one, drop the other. On the map, that’s the corridor.
What goes wrong when you mix them?
- Low nutrients, high KH
- Burnt tips. The skeleton outruns the tissue.3 Top left of the map.
- Phosphate stripped, nitrate still up
- The usual way to botch ULNS. Corals starved of phosphate with nitrate still in the water bleach more easily under heat and light.5 Bring them down together.
- High nutrients, low KH
- Browner and slower. Nitrate feeds the zoox and cuts skeleton growth.4 Bottom right of the map.
- Switching lanes in a week
- Every coral in the tank changes lanes at once, and swings hit hardest at the low end.1 Move over weeks, not days.
I’ve run lower KH with high nutrients under high PAR, and I’ve run it the other way around. Learned this the hard way, both directions.
What happens when a coral changes tanks?
If your tank runs a different lane from the one it came from, the coral changes lanes too, and it shows.
- From a low-nutrient tank into a classic one
- It may go darker before it settles. Darker is the zoox catching up to the food, not a sign it’s dying.
- From a classic tank into ULNS
- The harder direction: it just lost its food supply. Feed it, keep phosphate off the floor, and don’t chase color the first few weeks.
- Across a big KH gap
- A coral going from 8 dKH to 11 in one afternoon is changing lanes in a day. The closer your numbers sit to the tank it came from, the less it has to adjust. Plot yours on the map to see the gap from mine.
Where I run
On the border of ULNS: KH 7.5–8.0, nitrate 0–5, phosphate 0–0.03. My phosphate sits inside ULNS, my nitrate runs a little over it, and my KH sits at the top of the ULNS range to match. It’s the white bar on the map.
What I dose: kalkwasser, Korallen-Zucht aminos and Pohl’s Xtra. That’s the whole list.
Every number is on the System Parameters page, and what to do the day your box lands is in the acclimation guide.
See what came inSources
- Practical Fishkeeping. Frequently asked questions on Ultra Low Nutrient Systems. ULNS targets (nitrate 0–1 ppm, phosphate under 0.03, alkalinity 7–8 dKH), what the method runs on, and why it is not for beginners. practicalfishkeeping.co.uk
- Korallen-Zucht, KH Plus Concentrate (product listing, Bulk Reef Supply). Korallen-Zucht recommends holding KH between 6.5 and 7.5 dKH on the ZEOvit system. bulkreefsupply.com
- ATI North America (2020). Water Quality for Reef Tanks (Part 1): The Most Important Parameters. Alkalinity run too high can outpace tissue growth and burn SPS tips. ULNS belongs at the low end of the alk range, higher-nutrient tanks at the high end. atinorthamerica.com
- Marubini, F. & Davies, P.S. (1996). Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Marine Biology 127, 319–328. Adding nitrate raised zooxanthellae density and cut skeleton growth. doi.org
- Wiedenmann, J. et al. (2013). Nutrient enrichment can increase the susceptibility of reef corals to bleaching. Nature Climate Change 3, 160–164. High nitrogen with starved phosphate makes corals bleach more easily under heat and light. doi.org
- D'Angelo, C. et al. (2008). Blue light regulation of host pigment in reef-building corals. Marine Ecology Progress Series 364, 97–106. A coral's own pigment is tightly controlled by light, mostly the blue end. doi.org
- Bollati, E. et al. (2020). Optical feedback loop involving dinoflagellate symbiont and scleractinian host drives colorful coral bleaching. Current Biology 30, 2433–2445. Fewer zooxanthellae lets more light into the tissue, and the coral answers with more of its own pigment. doi.org