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 to 8.0-8.4 indefinitely; aquaponics nitrification slows above pH 7.5 and stalls above 8.0, so this medium fights the system
- Iron becomes biologically unavailable above pH 7.4 regardless of what's in the water, so plants chronically run iron-deficient
- Continuously releases calcium and carbonate as it 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 that's running too acidic from heavy fish waste; once stabilized, the limestone has to come out
- People sometimes find this material cheap or free as driveway or landscaping gravel and use it without realizing it's calcium carbonate; the vinegar test catches it (a drop of vinegar fizzes on limestone, does nothing on inert gravel)
Notes
If you've already filled a bed with limestone and the system is struggling, the only fix is to drain, swap the media for an inert aggregate, and re-cycle. Mixing limestone with inert media doesn't help; even 20% limestone is enough to drive pH high.
Sources
wilson-lennard-aquaponics-handbook, uvi-aquaponics-program
See the full aquaponics media reference for comparison, or use the aquaponics system designer to plan a complete setup.