Phosphates most usually come from fish food. Look at your food container - it will say something like "phosphates not more than 9%", or "max. ash content 9%".
There is a test from Aquarium Pharmaceuticals.
http://www.aquariumguys.com/phosphatetest1.html
I have used it a few times but I do find it hard to match the colors. It seems to be accurate at low levels - at 0, .5, or 1.0 ppm, it looks accurate, but sometimes it shows a color that isn't even on the card. At that point I figure my level is too high and I do a water change.
Phosphate levels should correspond to nitrate levels like this: At 5 ppm nitrates, your phosphate level should be .5 ppm. At 10 ppm nitrates, the phosphate level should be 1.0 ppm.
Your lighting is low, so maybe that's why the plants aren't using the nitrates faster, and your level is a bit high. I try to keep my levels at 10 ppm nitrates and 1 ppm phosphate. If your nitrate level is higher, your phosphate level needs to be correspondingly higher. I don't know that a level of 30 ppm nitrates is necessarily bad unless you have sensitive fish like otos. I have had nitrates up to 50 ppm, with phosphates of 4 ppm, with no algae issues. I had
no fish in the tank at that time!
The only way I would recommend to reduce phosphates is to do more frequent water changes, don't feed as much or try to find a food with less phosphate. Are you overstocked by any chance? I would not use the phosphate-removing chemical media in a planted tank.
PS. While it's probably not likely, your tap could have high phosphates. Do you live in an agricultural area - runoffs from farms and fields could get into your water supply, possibly, if you have well water.