After the first flush diverter fills up, the water pressure will then push the rainwater through the side leg to the holding tank. I used an intermediate bulk container (IBC) that holds about 250 gallons. It was food grade product and I washed it out several times to get the best quality water. The IBC is wrapped in several layers of black plastic to keep down the light so no algae will grow. As the downspout enters the IBC, I put a Y on the pipe and covered that with a mesh paint strainer bag. That way, when the water displaces the air in the IBC it has an exit and when the IBC is full, it acts as an overflow. The bag prevents vermin or insects from contaminating my water