Calcium carbonate is a chemical compound found in limestone, eggshells, sea shells and a few other things. Calcium carbonate, usually limestone, is ground into a powder to neutralize acidic soil (soil with a low pH). Once soil pH goes below 6.0 (acidic) or above 8.0 (alkaline) it becomes difficult for most plants to absorb the nutrients they need to grow. Chances are that the potting soil you used for your plants has a pH of 6.5-7.0 which is optimum for nearly all plants. Also, it’s likely that the crushed coral is not fine enough to make any difference in your soil this year. If you want to put a small amount in each pot and on your garden it should be fine. The calcium carbonate will leach out slowly and should do no harm. Hope your turtle enjoys his nice new home!

For more information on calcium carbonate you may choose to read any/all of the following: https://ohioline.osu.edu/factsheet/hyg-4026
; https://www.science.org.au/curious/earth-environment/sea-shells
; https://www.esf.edu/pubprog/brochure/soilph/soilph.htm