I wouldn't recommend API Phos test, The only one I have found to be accurate is the salifert brand. But I'd first say to take a sample to your LFS to verify your test results. It sounds like you're taking all the right steps, but from the pics I don't really see any algae in your tank. If it's just not visible from the pic, then I would want to ask the types of food you are feeding. And if you are doing water changes to eliminate extra nutrient consumption.