I looked into using Phosphate Rx but I was leery of the manufacturer's "easily filtered out" claim regarding the precipitates that will form. I don't have a filter sock or filtration system that would capture the precipitates and wasn't sure how to go about rigging something up. Maybe the precipitates aren't that big of a deal anyway

. I wasn't able to find a solid answer on that question except for it being bad for Tangs.
I would love to hear how it works for you though on your 13 gallon, and would appreciate your feedback. I will probably have to do one more round of phos control to finish balancing my rock.