On an episode of ATOH, Tom helps a homeowner clean and stain a deck. After cleaning, and prior to staining, he recommends spraying a water/borate solution onto the deck to prevent algae from coming back. On the show it appears to be a thick viscous liquid (maybe a gel). I've called a bunch of hardware stores and also checked Home Depot and cant find this borate.

Does anyone know where you can buy this liquid borate?

Thanks in advance.