One thing that I love about this hobby is that the addicted reefers love to share there addiction. Youse guys are so much more knowed up then when I started this hobby.
Liz, the substrate can have phosphate also. Some inorganic forms of phosphate will not show up on most test kits. Use the red algae as an indicator and you don't need to test so often. Some forms of Caulerpa have the highest ratio of phosphate to nitrate, by as much as 10 fold. Food is always a source of phosphate. Unlike nitrate, which has de-nitrification (anerobic digestor in a waste water treatment facility) in our deep sand beds and the interior of the live rock. We do not have bacteria to remove phosphate. I do not consider "dilution as the solution to pollution". At best it is a bandaid. It is expensive with a large system. I prefer multiple nutriant pathways. Chemical removal with a phosphate resin, in my opinion, should be continuous as should be the use of activated carbon. For me it is mandatory because I do not use a skimmer nor do I do partial water changes. Algae is inevatable on the coral reefs of the world. Thus the reason for the herbivors on the reefs.
Just my $.02 worth.
Patrick