The size of your pump doesn’t depend on the size of your reservoir; rather it depends on how far you need to pump your water and how much water you need to pump.
You want to avoid overworking your pump, so in choosing the proper pump you will want to choose one with at least 20% more power than need. To find out your appropriate pump size you will need to determine how much water is necessary to fill your tray. If your tray is in the shape of a rectangle or square then you will need to apply the following formula to determine its volume:
Length (ft) x Width (ft) x Average Depth (ft) x 7.5 = ? US gallons
This will give you the total gallons that your tray can hold. It is a good idea to always get a pump that is at least 20% larger than necessary to avoid overworking it.
After you’ve determined your volume requirements you need to find out how far “up” the water needs to be lifted in order to reach the tray.
Simply measure the distance between your pump and the entry point in your tray; most systems will have a distance of under 3’. This vertical distance will have an adverse affect on the pump and this affect must be accounted for. In essence, the greater the vertical distance the water must travel, the stronger the pump needs to be. The following chart will show you how vertical distance affects the pumps. Note the loss of power of each pump as the vertical height increases.
Pump Size (GPH)
1ft 3ft 5ft 7ft 9ft
120 120 70 40 x x
170 170 130 70 x x
205 205 170 120 40 x
300 300 250 200 160 110
500 500 350 280 200 150
700 700 520 350 280 200