Often they are found on top of the tank waving at you with their little fiddler arm.
Start a grid search from the tank out. They hide well, also on and behind any tubes you have up above the water line.
They WANT to be in the air, so check into one of these reptile turtle lily pads, are pretty reasonably priced.
Duct tape and doubled over heavy duty plastic wrap is what I used. They can get out of the tiniest of spaces.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves