Not sure if that many/size fish in the tank will provide enough nitrates for the plants but beforing adding any, test for it; an issue you need to be aware of is phosphates - both the waste food and fish waste will build these up in the sand over time so even after heavy water changes, this will be a problem. Plants do not remove this chem very fast and many ferts include more of this bad guy! Avoid all solid/under sand ferts in a discus tank (shot gun types) but rather, add only what your plants need after testing (like Fe, for instance.)