Get creative with tank decor. You can use the floating items trick to sink the Stuffed Takitaro or Stuffed Mazlaya Marlin in the tank so it looks like you have a large fish without using up one of your fish slots. Put a low table under the aquarium so you can put table top decorations inside to take up space. Vigil Piece looks like a cute sandcastle, Dead Man's Chest seems like an aquarium staple. You can sink the flower pots and planted partitions in the tank to make it look more densely grown. Creativity is king.