You can always send decorations you don’t currently need to temporary storage:
1. Tap a decoration to open the menu below it.
2. Tap the box icon in the menu and the decoration will disappear.
3. You can find the decorations you’ve removed in the Store, where they’re marked Stored.
Once you decide you want a decoration back:
1. Open the Store tab.
2. Tap the decoration you’re looking for.
3. In the next window, tap the Place button to enjoy the decoration in your aquarium.
You can’t store your fish, though you can always sell them.