Building an aquarium, I'm using Create's Hose Pulley to fill up areas with water.

A friend of mine used 2 slabs on top of each other as wall, instead of using normal full blocks This made a combined (full) block from this mod.

The Hose Pulley will waterlog every waterlogable block, meaning all water comes out of the aquarium if double slabs are used.

The doubleslabs:mixed_slab block doesn't have the waterlogged property on the most recent version, so I'd imagine this is fixed.