Giant Shrimps!

The biggest sized shrimps you see in the RH Fish Tank are...
Singapore Wood Shrimps (Atyopsis Moluccensis).

They are NOT "prawns".

There are two Wood Shrimps, and each have four filter-like limbs with which to filter out algae/microorganisms to eat.

They aren't very active, and can usually be seen standing around being greedy, putting its feeding limbs into its mouth every few seconds.