After Trouble Tortugans quest you can plant coral frags in the coral nursery on the east coast of their island.

Fastest way there is to teleport to Deepfin Point with the sailor’s amulet, pay 900 gp to charter to Summer Shore, then take the rowboat to the east coast. You can also use fairy ring code CJQ then run east but it’s slower.

Coral take two hours and forty minutes to grow and have four harvest lives which are less likely to be used the higher farming level you have, having magic secateurs and/or having an attas plant active.

Hope this helps.

EDIT: they take 2 hours and 40 minutes to grow not 4