The best replacement for coral calcium is calcium citrate.

The best alternatives to coral calcium are rated as following:
★★★★★  calcium citrate
★★★★  calcium
★★★  calcium carbonate
★★★  regular calcium
★  oyster shell calcium
★  citrate
★  other calcium
★  oyster calcium
  oyster shell
  algae calcium
  citrate calcium
  elemental calcium
  oyster shells

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Speaking about coral calcium, one probably shouldn't forget about alkaline.

Other search terms related to coral calcium and their ratings are:
★★★★★ alkaline
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coral calcium
calcium citrate
calcium