Don't think you need to isolate the tanks as they are below the level of the pump. so when you disconnect the pump the water in the pipework will drain down into the tank that is open. As soon as you reconnect the pump it will draw water up from the tank. Good idea though to put a shut off valve in the feedpipe from each tank as they drain by gravity. No need to attach it to the tank itself - it can be close by in the pipe. I used normal domestic plastic valves for mine. You have to drain the tank first of course. I also put shut off valves in the deck shower feeds, located in the lazarette.