One of the fluridone brands would likely be the best option, assuming the brittle naiad hasn't fully matured. Even so, you will probably need two consecutive annual early-season treatments before long-term (multi-season) control will be achieved.
Dosage: 1 qt of liquid or 20 lbs of SONAR ONE (pellets) should do the trick - based on your stated pond-measurements.
Aquathol Super K might also work. However, I'm guessing your pond-water might be a little brackish, since brittle naiad is usually partial to such conditions. If so, Aquathol (endothall) doesn't work very well, if at all, in brackish water.
