All the same, dump the res, clean the tub, put new water & nutes in, add hydrogen peroxide as was suggested, put in an airstone. It's difficult to ascertain just looking at that picture the exact maladay you're suffering but the advice given you is all good regardless of the problem. After taking these steps it's possible this problem will go away. If not, and the plant suffers and is dying, the humane thing to do would be to lay it gently to rest and start another seedling. If one plant has a problem like this, chances are any other plants near it will be susceptible as well.
I spent several months growing various types of vegetable plants in different hydroponic setups before settling on one particular style. I wanted more experience under my belt before going through the trouble of getting seeds and starting a good grow. Maybe you should try some peppers, tomatoes, peas, etc. to get some more experience. Experiment with growing and learn from your mistakes as well as your successes. Once you've got a feel for growing, your grow setup, your grow space, etc. then pop in some magic seeds and see what happens. Don't be discouraged if your plants just aren't doing their thing, man. This is how we learn. The more of us who take the time and effort to learn, the fewer of us there will be attached to the strings of the government anti-cannabis puppetteer.