High Phosphates in Rinsed Sand

[Werbo]

So more than a year ago I went bare bottom. Took the sand out and rinsed and rinsed and rinsed it till the water ran clear. Dried it out and stored it in Rubbermaid in the garage.

Decided I wanted to add sand to give the tank a finished look. Rinsed the sand again with hose and drained the fresh water out. Just to make sure I mixed up some new salt water and threw a heater and powerhead with a filter on it into the Rubbermaid.

After 2 weeks I pulled out my Hanna checker and phosphates were 0.93

Is this normal? Will cooking the sand in the dark and doing weekly water changes bring down the phosphates?