I have a 10g planted tank with a 2" layer of flourite for substrate. I have 9 1" cardinals in there along with 8 or 9 Crypt. wendtii. I'm having a wicked time with blue-green slime algae (cyanobacteria) and have been since starting this tank up about 7-8 months back. The crypts aren't doing well because I am constantly removing the BGA from them, along with the substrate, the heater, the filter, the glass, pretty much everything but the cardinals. This tank has a 15w fluorescent (the standard "All Glass Aquarium Lamp" that came with the lighting fixture) light. The filter is an AC20 with foam and ceramic media and 100ml of Purigen. I have an airstone running in the opposite back corner to add some movement. This morning was the 4th day in a row that I did a 75% water change trying to lower the phosphate level. Soon after the water change I get around 1 ppm for a phosphate reading. The next morning before the next 75% wc I get 2 ppm phosphate. Another odd thing is the pH in this tank is 7.5, while the tap water I've been adding comes out of the faucet at 6.8. I add nothing and I've tested my tap water for many things and I haven't found a reading on any of it. This is how the water stands after the fourth 75% water change, Temp 76 degrees, hardness 3 dKH, nitrite 0 ppm, nitrate 0 ppm and the phosphate nearing 2 ppm. Questions: Is the flourite adding the phosphates? Could the Purigen be removing something from the water to cause the higher pH? The Purigen I added just a few hours ago is already much darker showing it's pulling something out and quite a bit of it at that. Maybe I should pull the Purigen out? Any ideas guys?
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