One used Eshopps Float Valve.

Eshopps’ Float Valve offers a simple solution to help maintain proper water level. Use the Eshopps’ Float Valve as part of an automatic top-off system you create to replenish aquarium water lost through evaporation. Adjustable acrylic mounting frame allows water level adjustment for versatile installation options. Features nylon screws (no metal parts) for corrosion and rust-free use.