Tankful

An aquarium is a block that players build and fill with water, and fish placed inside it swim about freely. Each fish carries its variant with it, so a placed axolotl keeps its colour and a tropical fish keeps its pattern. Placing aquariums next to each other merges them into one volume, and the glass between them disappears so fish can swim across the boundary. Aquariums can be stacked, fanned out into a U-shape or built into a 27-block cube wall, and whatever shape is built, the fish use it.

The two decoration items are a shipwreck and an ocean ruin. Each drops a miniaturised version of one of the vanilla underwater structures into the aquarium, using real shipwreck variants with the mast sideways, a full hull or a broken one, and real cracked mossy brick and warm-stone pieces for ruins. Both turn up in the same loot chests as their vanilla counterparts. Players can place any block tagged tankful:plants as substrate, and kelp, seagrass and coral are included by default. Datapacks can extend that list. Craft fish food using the recipe in the mod, then right-click an aquarium to release a few drifting orange motes; the fish notice them, swim over and eat.