The algae is consuming the Phosphates as fast as it builds up so you will get a zero reading on the phosphates. Have you considered getting a sand sifting goby to clean the sand. They do an amazing job of turning over the sand and keeping it clean, but I'm not sure if your system is big eneough for one.

Hmm, any other ways around this?

I'm curious, I don't think anyone has asked about your source water. Do you have an RO/DI unit, or do you buy filtered water, or use tap water, or what? If your phosphate levels read 0 and you're feeding sparingly, there's got to be something feeding the algae.

I am using RO water, and dont have any fish in the tank, so I feed a tiny bit once a week or so. I am very confused.

how often do you do water changes? personally i would remove the sandbed completely, i find its much easier to clean!