River gravel (pea gravel)

Properties

In a system

• Smooth, round, low-porosity stones at roughly 1500-1700 kg/m3 flooded; the heaviest common media by a wide margin
• Bacterial surface area is only about a fifth of LECA, so beds must be larger in area to handle equivalent fish loads
• Compacts in the bed over time; ebb-and-flow drainage stays even but deeper beds need regular probe inspection
• Cheap and locally available almost everywhere; builders often use it to save on the initial fill at the cost of a larger bed footprint
• Vinegar-test a sample first: gravel that fizzes when vinegar is dropped on it is limestone (calcium carbonate) and will raise pH, whereas silica/quartz/basalt river gravel is inert

Notes

Source it from a landscape or aggregate supplier in 6-12 mm size. Avoid coloured or coated gravel from pet stores, since coatings flake over the years, and rinse thoroughly because landscape gravel often arrives dusty. Whether river gravel is inert or pH-raising depends entirely on its rock type, which the vinegar acid test reveals.

Further reading

• Building a media-bed aquaponics system
• Aquaponics water chemistry: balancing pH for fish and plants