what's the deal with phosphates?

[stanzi]

I've been fishkeeping for about a year now. You follow advice and learn as you go I guess. Anyway I wonder if any of the big brains can answer a question for me. I'll give you some info pertinent to my question. Oh firstly I love this forum. That said here's the deal. I have a forty five gallon tank for now. It has 7 barbs, 4 platies, 2 cory's, and 1 small chinese alge eater. -So the tank is not overstocked. For filtration I have a Filstar cannister medium which is recommended for up to 70 gallons. In addtion I don't over feed. All that said. Now for the water- It is well water. I live off of the lower Potomac river right off of the Chesapeake bay. The well is very deep. I would venture to say approx 200 ft. The land was at one time a cow farm. We are sparsely populated there have never ever been a whole lot of people or animals here ever. The natural ph is 8.2. (I reduce the ph to 7.4) My substrat is sand (instant aquarium from carib-sea) so I don't have a lot of gunk seeping down into gravel. I have always had a devil of a time with nitrates. I do a 25% water change pretty much weekly. With that said usually the nitates by that time are up to the 40ppm range. Ive used nitate reducers (Fluval) and it works ok for a while and then it spike and dies after about 5 recharges. However, the phosphates in my water are off the charts. When I tested them with the API kit the water turned saphire when the first drop of bottle 2 hit the tube. The folks at the LFS said they don't even test for phosphate so I disregarded the whole issue--until yesterday. I added an API phos-zorb pac to my filter. 24 hours later my phosphates are still off the charts. But wow my nitrates are 0! Why did this happen?