Limestone gravel
Also known as: Crushed coral, Calcium carbonate gravel, Aragonite, White driveway gravel
Avoid in aquaponics
Properties
| Bacterial surface area | 80 m² per m³ |
|---|---|
| pH effect | raises pH permanently |
| Weight class | very heavy |
| Longevity | indefinite |
| Cost tier | low |
In a system
- Buffers pH upward toward 8.0-8.4 indefinitely; aquaponics nitrifying bacteria are most efficient near neutral (optimum around pH 7.3-7.5) and slow at higher pH, so this medium fights the system
- Iron becomes largely unavailable to plants in alkaline water (high pH), so plants chronically run iron-deficient (chlorosis)
- Continuously releases calcium and carbonate as the calcium carbonate dissolves, so water-quality readings drift over time and supplement decisions become unreliable
- The only legitimate use is short-term emergency buffering of a system running too acidic from heavy fish waste; once stabilized the limestone must come out
- People sometimes use it unknowingly as cheap driveway or landscaping gravel; a drop of vinegar that fizzes confirms calcium carbonate, while it does nothing on inert gravel
Notes
If a bed has already been filled with limestone and the system is struggling, the only fix is to drain it, swap the media for an inert aggregate and re-cycle. Mixing limestone with inert media does not help, since even a small fraction of calcium carbonate is enough to drive pH high. Crushed coral and aragonite are the same carbonate chemistry.
See the full aquaponics media reference for comparison, or use the aquaponics system designer to plan a complete setup.