Are you 100% convinced the ply will survive the damp and water, especially once the plants cover it ? Varnishing ply in a damp environment is a no no as well as the wood can no longer breathe and black mould patches will form under the varnish.
Local shop had a similar tank with plant covered back wall and they used waterproof boarding designed for bathrooms. Expensive stuff is expanded foam with formica decorative finish and cheaper is resin bonded MDF with Formica decorative finish. All 100% water proof and easy to drill for screws to attach netting and plant holders to keep the plants under control. Also used acrylic sheet designed for secondary double glazing as a back wall.
Local shop had a similar tank with plant covered back wall and they used waterproof boarding designed for bathrooms. Expensive stuff is expanded foam with formica decorative finish and cheaper is resin bonded MDF with Formica decorative finish. All 100% water proof and easy to drill for screws to attach netting and plant holders to keep the plants under control. Also used acrylic sheet designed for secondary double glazing as a back wall.