not sure how bad it will be but you have seen rice water ahen cooking lots of free floating organics (the water turns white), fortunatly rice mostly absorbs water so once you siphon it out most of it will be gone, your skimmer should take care of the residue, just be prepared for an ammonia/nitrate spike due to the influx of excess organics, watch your water closely (I'm sure that goes without saying)
hopefully none of your fish injest the rice before it swells it could cause rather serious gastrointestinal problems, like ruptures internally.