If your nitrites are high, it means the system hasn't cycled completely - do a bit of reading on the Nitrogen Cycle to understand. There are a few things you can do, other than water changes, that can lower nitrites:

1) Feed less. On a new system, that doesn't have established bacteria colonies should really only be fed once per day. Once you've started getting 0 Ammonia, 0 Nitrite and raising Nitrates, your bacteria colonies have established and you can slowly increase the ammount of feed.

2) Plant more! Get some plants with healthy roots going if you can. Cuttings from houseplants are a good way to start if all you have are seeds to start your veggies. IME, DWC systems seem to help get roots going fom cuttings the quickest, and if the plant doesn't like being constantly wet, move it to a Flood and Drain system once the roots are established.

3) Add heterotrophic bacteria. In the pond section of big box stores and greenhouses (in my area, anyway), you can find dry bacteria (often called something like "sludge buster") to add to your system. If you can't find it there, you can usually find a liquid form in the fish section of most pet stores - if you have a store that only deals in aquariums, go there, as they typically have better products available. If you can't find it anywhere, ask a friend who has a fish tank or aquaponics system for some of their media. A good squeeze from a dirty sponge filter will give you a good dose of bacteria. Either add it to your FT right at the pump, or even better in your filter/flood & drain.