This may help you too. A 20 gal res is a lot for those plants. It wont hurt them at all, but it will take them a very long time to use all the nutes in 20 gal. In that time, your water will get dirty and pH may go nuts. So you may change the water sooner to keep it clean, but that is wasting nutes which is wasting money.

I recommend a 5-10 gal res for this time, once the plants are bigger they will drink a lot more. Doing this will save half your nutes. I hate buying nutes, I wish I had the money for the 5 gallon jug of GH 3 part.