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The Magic Triangle

Light, Alkalinity (KH), and Phosphate: Why the balance is more important than individual values.

Glossary / Definitions

  • ECF (Extracellular Calcifying Fluid): The microscopic gap between tissue and skeleton where calcification takes place.
  • DIC (Dissolved Inorganic Carbon): Dissolved inorganic carbon ($CO_2$, Bicarbonate, Carbonate).
  • LEC (Light Enhanced Calcification): Calcification boosted by light (up to 10x faster).
  • Aragonite: The crystal form of calcium carbonate from which corals build their skeleton.

1. The Interaction

You cannot view any value in a reef aquarium in isolation. The growth and health of corals depend on the balance between Energy (Light), Building Blocks (KH), and Nutrients (Phosphate).

Light – The Engine

  • Function: Primary energy source for photosynthesis and the driver of calcification (LEC).
  • Too low: Energy deficiency. The coral stores more zooxanthellae and turns brown (darkening).
  • Too high: Radiation stress. Risk of "sunburn" (oxidative stress) if there are not enough nutrients for repair.

KH (Alkalinity) – The Building Block

  • Function: Supplies bicarbonate as a precursor for the skeleton and buffers the pH value.
  • Too low: Substrate deficiency. The coral must expend extreme amounts of energy to maintain the pH in the ECF. Growth stagnates.
  • Too high: Creates chemical pressure for crystal formation. Danger: If nutrients are missing, the skeleton grows faster than the tissue ("Burnt Tips").

Phosphate ($PO_4$) – The Regulator

  • Function: Essential for tissue construction (cell membranes) and energy transfer (ATP).
  • Too low (Limitation): Tissue cannot grow. High Light/KH values lead to immediate tissue loss.
  • Too high (Inhibition):
    • Biological: Algae bloom in the tissue (coral turns brown) and competition for carbon.
    • Chemical: Phosphate deposits directly onto crystals ("Crystal Poisoning") and physically stops growth.

2. Deep Dive: What happens chemically?

Calcification takes place in the ECF. The coral pumps calcium in and protons out to raise the pH to over 8.5. Only then does aragonite precipitate.

The problem of over-fertilization:
If phosphate is too high, zooxanthellae multiply explosively. These algae consume all the inorganic carbon (DIC) for their photosynthesis. The coral (the host) lacks carbon for skeleton building, even if enough KH is measurable in the water. Additionally, phosphate acts as a "crystal poison," directly blocking the growth zone of the skeleton.

3. Practical Guide & Ratios

How do you set up your tank? The goal (SPS vs. LPS) and the balance are decisive.

Scenario A: "High Energy" (SPS Dominance)
  • Light: High (high PAR).
  • KH: Elevated (7.5 – 8.5 dKH) to cover high consumption.
  • Phosphate: Low but stable (0.03 – 0.06 mg/L).
  • Goal: Maximum color and growth.
Scenario B: "Low Energy" (LPS / Soft Corals)
  • Light: Moderate.
  • KH: Natural (7.0 – 7.5 dKH).
  • Phosphate: Slightly higher (0.08 – 0.15 mg/L).
  • Goal: Fleshy polyps, low maintenance.

The Danger Zones

  • The "Burnt Tips" Zone: High Light + High KH + PO4 n.d. (0.00).
    Result: Skeleton grows explosively, tissue starves and tears.
  • The Stagnation Zone: High PO4 (> 0.2) + High KH.
    Result: Chemical precipitation on equipment, but growth stop in corals due to crystal poisoning.