The Pastoria Gym is a large in-door swimming pool comprised of three levels and numerous floating platforms. Switches on the floor of the pool raise and lower the water level. To get to the Gym Leader, the switches must be hit in a certain order so that a path will be formed with the floating platforms.
The Pastoria Gym was featured in the anime episode A Crasher Course in Power!. Like most Water Gyms in the anime, the Gym's battlefield is featured as a pool with several floating platforms on it for non-swimming Pokémon.
Ash faced Crasher Wake in a 3-on-3 battle, which culminated in Ash's Buizel battling against Wake's powerhouse, Floatzel. Buizel was eventually able to defeat its evolved form, earning Ash the Fen Badge.
In the anime, Ash's Pastoria Gym battle was Ash's only Sinnoh Gym battle where he didn't lose a single Pokémon.