Try your cheap test kit.
Your fish food and fish could not add that much PO4 if they wanted to.
Why do you have Rowphos in your filter?
It's possible it's expended and leeched it out back into the tank.

That will not have any bearing on algae nor is a solution to any algae
problem with a planted tank.

Do you realize plants remove PO4 very rapidly?
I dose 3ppm of PO4 per week to my tanks.

I have no algae. I've dose PO4 for over a decade, no algae.
I'd reconsider your entire approach to algae.

Focus on plant growth, not algae.
Healthy plants = no algae.
So give the plants what they need.

Regards,
Tom Barr