Diatoms Reef Tank Complete Guide: Brown Bloom Resolution
Three weeks after adding live rock, that rusty-brown dust coating sand, glass and plumbing is almost always a diatom bloom — not hair algae, not cyano, and not a failing cycle. This diatoms reef tank complete guide walks through the silicate-driven biology, the typical 2-4 week resolution window, and the mistakes that drag the phase out for months. Gensou Aquascaping at 5 Everton Park has watched almost every new reef tank in Singapore pass through this stage, and the outcome is almost always the same once the basics line up.
What Diatoms Actually Are
Diatoms are single-celled photosynthetic organisms with silica cell walls, not true algae in the botanical sense. In reef tanks they appear as a fine brown film that wipes off glass with a gentle swipe and coats sand with a rust-coloured crust. They need dissolved silica, light and a trace of nitrate or phosphate to multiply. Once competing microfauna establish and silica inputs drop, they collapse almost overnight.
Why the Brown Bloom Appears After Cycling
New dry rock, fresh aragonite sand and any residual tap water contribute silicates for the first 30-60 days. Your cycled tank has plenty of ammonia-oxidising bacteria but no grazing microfauna yet, no established coralline, and low coral biomass consuming light. Diatoms exploit that vacuum. The bloom is a sign the tank is maturing, not breaking.
SG Climate Speeds the Timeline
Singapore’s 26°C ambient reef temperature pushes diatom metabolism faster than cooler climates. What runs 4-6 weeks in a European basement often wraps up in 2-4 weeks here. If your bloom started at day 21 and you have not changed anything, expect it to peak around week 4 and visibly recede by week 6. Tanks dosed with silica-rich top-up water — sometimes cheap bottled RO without DI polishing — stretch the phase indefinitely.
Silicate Sources to Audit
Walk through the inputs one at a time:
- RO/DI output — test TDS at 0 ppm; anything above 2 ppm means resin exhausted.
- Salt mix — reputable brands like Red Sea Coral Pro and Tropic Marin run low silica; generic budget salts from Carousell can carry silicates.
- Dry rock source — MarcoRock and Pukani leach for weeks; CaribSea Life Rock sheds less.
- Sand — aragonite is inert, but some budget bags carry silica sand contamination.
Fixing the source matters far more than treating the symptom.
The Wait-It-Out Protocol
For tanks under 8 weeks old, patience plus a turkey baster does most of the work. Blast sand and rock daily during the peak, let the filter sock or skimmer capture the dislodged particulate, and swap socks every 2-3 days. Wipe glass with a dedicated reef magnet — not a freshwater one that might carry copper. Resist the urge to dose GFO or scrub aggressively; killing the diatoms prematurely creates a nutrient vacuum that dinoflagellates will gladly fill.
Light Intensity Through the Bloom
Keep photoperiod modest while diatoms are active. Six to seven hours at 40 per cent intensity on a reef LED is plenty for the first two months. Running full SPS-level PAR during the uglies phase feeds the bloom without benefit, since you have no demanding corals in yet. Ramp intensity and duration only after diatoms recede and coralline starts spotting the rock.
CUC Timing for Diatom Cleanup
A modest clean-up crew introduced around week 4-6 accelerates the transition. Trochus and Cerith snails graze sand and glass efficiently; expect SGD 4-8 per snail at C328 or Reef Depot. Avoid dumping a massive CUC before food is available — they starve within a month. Skip Nassarius until you have fish feeding, since they are scavengers rather than grazers. One snail per 4-5 litres is a reasonable starting density.
When Diatoms Do Not Resolve
If the brown phase pushes past 8-10 weeks with no retreat, something is feeding them beyond the initial silicate pulse. Check RO/DI membrane age — they fail silently after 18-24 months in Singapore’s warm supply. Test TDS weekly. Reef Depot sells replacement DI cartridges for SGD 35-45. Swap to a different salt batch if your current bucket is close to expiry. Persistent diatoms that start showing snotty strings under the film are actually dinoflagellates, which is a separate and more serious problem.
Nutrient Balance Through the Transition
Resist the instinct to strip nitrate and phosphate to zero. A reef tank needs a trace of both — target 2-5 ppm nitrate and 0.03-0.1 ppm phosphate by month two. Ultra-low nutrient conditions kill beneficial competing microflora and open the door for dinos. Feed the tank lightly even before livestock arrives: a small pinch of pellets once or twice a week during the diatom phase maintains the food web.
Coralline and the Post-Diatom Tank
Once diatoms clear, you should start seeing purple and pink spots on glass and rock within another 2-4 weeks, assuming calcium sits around 420 ppm and alkalinity at 8-9 dKH. Coralline is the visual signal the tank has crossed out of the uglies. At this stage the system is ready for a first fish and a hardy starter coral. Rushing livestock in during the diatom peak usually just creates stress without any speed benefit.
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emilynakatani
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