Coral

A pickaxe takes it directly, and so does mining out the sand beneath it or simply dropping a block on top. That last one matters in reverse: coral breaks the instant anything occupies the tile above it, including the short varieties that clearly do not fill a whole block. Most plant furniture survives being built over. This does not, which catches people out the first time they lay a floor across a reef.

You do not have to keep returning to the ocean. It will grow on sand anywhere inside a band along the left and right edges of the world, and inside that band the height is free. A sand platform with water on it works at the surface, in the sky, even up in space. Build the farm out at the edge once and stop making the trip.

The rate responds to conditions. Deeper water grows it faster than a shallow puddle, rain and wind both push it up, and open vertical space above the sand helps up to a point, so a tall column of water and air over your sand beats a cramped one. Crowding is what kills a farm: put pieces too close together and the rate collapses, and past a small number in one area it stops entirely. Seashells count against that limit too. Spread the sand out, leave gaps, and harvest regularly so the farm never chokes on its own output. You get an audible thump each time a new one appears.

For most players the Sonar Potion is the reason to farm it at all, since knowing what is on the hook before you pull changes fishing completely. Past that it feeds a small building line, Coralstone Blocks and Coral Torches and Reef Blocks, plus one hardmode helmet. None of it is urgent, so a modest farm covers a whole playthrough.

Placed coral picks its appearance at random from six variants, so a decorative row will not match and there is no way to force one. It does take on the paint and coating of whatever it is growing on, which is the only styling control you get. Actuating the block underneath will not knock it loose, but you cannot place it on an actuated block in the first place.