Just wanted to pass on a little info I used when I built my Ebb and Flow grow system. I was able to build my own system for way less money than buying a ready made one. After you decide on a flood tray , you will need a resevoir. Here is a formula to figure out what size you need.
Once you have chosen the tray size you will then need to size a
reservoir for the tray. The calculation for this is quite simple: Height you wish to flood in the tray (H) x Width of the tray (W) x Length of the tray (L). For this calculation it is best to convert feet into inches. Then multiply the cubic inches by 0.0043 to convert it into gallons.
Example:
For this example lets use a 3 x 6 Botanicare tray. The plants will be growing in 6x6 rockwool cubes, thus the tray will flood up to 4.
Reservoir size required (gal) = ((3x12) x (6 x 12) x 4) x 0.0043
= 36 x 72 x 4 x 0.0043
= 45 gallons (minimum)