Crushed Coral Substrate Guide
How crushed coral raises pH and KH to buffer hard-water tanks, which fish benefit, and how to use and replenish it.
What it is
Crushed coral is a calcium-carbonate substrate that slowly buffers pH and carbonate hardness (KH) upward. Because it contains calcium carbonate, it raises water hardness and pH, which suits hard-water and high-mineral setups rather than soft-water tanks.
How it buffers water
Crushed coral adds hardness and notably boosts KH, the carbonate buffering capacity that resists pH swings. Low KH means water buffers poorly and pH can swing or crash easily, while higher KH holds pH stable. Aquarists with low-KH tap water use crushed coral to gradually raise KH and prevent pH crashes.
Which fish benefit
Crushed coral suits African rift-lake cichlids, livebearers, and brackish setups, which prefer harder, more alkaline water. Rift-lake cichlids in particular appreciate KH above about 10 dKH (around 180 ppm), which tends to come with higher pH.
When it is not the right choice
Because it raises hardness and pH, crushed coral is not appropriate for soft-water species such as many tetras, discus, or crystal shrimp, which prefer low GH and KH. For those tanks an inert substrate is used instead, so that the keeper controls hardness separately rather than having it driven up by the substrate.
How it is used
Crushed coral can be mixed into the substrate or placed as media in a hang-on-back or canister filter. A common starting point is about 1 pound per 10 gallons of water (roughly 0.45 kg per 38 litres) to raise pH, KH, and GH, adjusting the amount based on testing. Using it as filter media has the advantage that it can be removed or topped up easily without disturbing the display, while mixing it into the substrate keeps it out of sight.
Dissolution and replenishment
Crushed coral dissolves over time as it releases minerals; the lower the pH, the faster it dissolves. As a result it gradually depletes and is typically replaced about every 6 to 12 months to keep remineralising the water.
Monitoring
- Test KH and pH before and after adding crushed coral to track the effect.
- Add it gradually rather than all at once to avoid sudden parameter shifts.
- Allow time for the parameters to stabilise after a change.
- Top up or replace the coral when KH and pH begin to drift downward again.