Spa_builder

Hi I am designing my own spa made from fibre glass... I have sourced most materials and made a "tub" in GRP but I am unsure on how most spas design the overflow. I am planning on fitting a number of overflow outlets, possibly a skimmer fitting at the desired maximum height of the water. This will be connected by plastic pipe to the top of a water tank - possibly a standard plastic water tank from a plumbers merchants, a seperate plastic pipe would be connected to a pump, the pipe would go through a filter and then be fixed approx 2" from the bottom of the tank, the water would then be pumped back into the spa at varous points around the spa. A water level switch would knok the punp off if there was insufficient water in the tank, non return valves would prevent water going back into the tank if the pump stopped. The principle being that displaced water would flow into the overflow tank, this would then be filtered and pumped back into the spa. My questions are... 1. How do commercial spa manufacturers manage overflow / water displacement 2. How do I calculate the size of the overflow tank, is there a basic guide ie x litres per person in the spa 3. Does anyone manufacure overflow tanks or fittings Any other advice or information much appreciated Thanks Harry