Hydra Pools HSS3M04179 4' 3-Tread Steel Step Sub Assembly - This Steel Step Sub Assembly is for use with Hydra in-ground pools. It is constructed of durable steel. Made for easy installation and use, it will help ensure the best service life possible. 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.