Holding The Water (May be a daft question)

[Mentat]

Normally the water level would be same throughout enclosure and kept below substrate level by egg crate. If you plan a water opening and do not want critters, etc. from reaching beyond the opening; recommend use same botanical netting material you use above egg crate to separate from substrate, and create boundaries while allowing some water flow. If interested in totally isolating the water opening; then you can use glass and silicone glue to build a dam. Good luck !