1) Remove livestock.
2) Drain the tank.
3) Rescape, misting the plants often.
4) put a plate over the substrate, partially refill, making the water run over the edge of the plate and seep slowly into the substrate. A plastic bag will also work.
5) Drain this, vacuuming the surface substrate.
6) Put a plate over the substrate... ...and fill the tank. Don't forget the dechlor.
7) Turn on the equipment when the water is high enough.
8) Add the livestock back.