[tang daddy]

What you stated reads pretty good.... Although I have some questions.

What size tank?
How much gfo do you run?
What lights are you using and how old are the bulbs?
How much water is replaced every 3 weeks?
Which test kit are you testing with?

I find the phosphate is very hard to test, I used a Hanna and it came up as 0 but I still get algae growth on the back glass, in the sump and on a few pieces of older LR that leech... The rest of the rock and sand is clear.

There are usually a few factors to consider when you have algae problems.



One of the biggest factor is lights, if the bulbs are older the spectrum is shifted and you will grow algae well. I have old t5 bulbs and know this is adding to some algae growth. The next is nutrients, wether from your live rock being saturated with nitrates and feeding....
The third is the water Coloumn, if you're doing a 10g water change on a 65g every 3 weeks it's not enough.... Over time the nitrates will build causing more algae growth.

Also why run filter floss, and how often do you change it?
Filter floss can build nitrates fast, you should be replacing or rinsing it well every 2-3 days, if it were me I'd ditch it.
If you have detritus settled in the sump, remove your equiptment and siphon it out when you do a waterchange.

Sit back take a deep breath and visualize what is happening... The answer is to your problem is in your tank.