Hydra Pools HP504 4' Tranquility Panel for In-Ground Pool - This panel is for use with a Hydra in-ground pool. Inside and out, only the finest goes into Hydra pools. Our precision engineering means that every part is meticulously designed and fabricated for flawless fit and finish. The strength of Hydra's intelligent design means you will have a swimming pool that you can enjoy trouble-free for years to come.