Each school of fish enters at its dashed square on its beat (the number on it counts down) and swims one cell every beat. Arrows in the water push it their way; in still water it keeps swimming straight. A school that meets a wall turns right if it can, then left, then back.
Get every school to the bed of its own colour and number. Schools swim past other colours' beds. Two schools in one cell merge and follow the bigger one; each still drops off at its own bed.
Gates (A, B) are vents you turn before you release: tap to rotate. The tide flips marked gates half a turn on the beats shown in blue on the timeline. The moray shuts its den on the red beats. The shark swims its loop and chases any school it touches back to its entry, unless the school was calmed at a cleaner station. A sea-fan baffle blocks a cell; a kelp eddy makes a school circle for 3 beats.
The dashed lines show where your plan will send every school; a red cross marks where a school ends up lost. Drag the timeline to see any beat. Cost = pieces placed + gates turned (lower is better): any clear is bronze, then silver and gold. Ghost swims your best plan beside you; Hint sets up the first steps of a known plan.
Keys: arrows move the cursor, Enter or X turns a gate or places, Delete lifts, 1/2/3 pick baffle, eddy, cleaner, 4 lift, Space or R releases, [ ] scrub, Z undo, G ghost, H hint, F speed, C clear, Esc reefs. Pad: d-pad, A place/turn, B lift, Y release, X ghost, LB/RB pick a piece, LT/RT scrub, Start reefs.