I think its a bacterial bloom, and will go away on its own. Often, when ammonia levels become high, the bacteria colonies grow in relation, which causes cloudiness. However, when ammonia levels become stable, so do the bacteria colonies. Also, 100GPH is quite a bit for a 3 gallon, a 300%+ turnover rate. It may in fact be the sand being kicked up if the outflow is reaching the bottom. You could try decreasing the flow by using filter floss or a cut up water/soda bottle and wrapping the outflow, to disperse the water.