How are you controlling your pump? From what I read it sounds like just with a thermostat so it's on or off.
I built my own controller and use a central heating circulating pump.
To maximise solar gain I control the pump by turning it on and off with varying duration depending on the temperature difference between the tank and the collector.
i.e. if the difference is only 8 degrees then the pump is on 8 seconds, off 2 seconds. When it's 3 degrees it's on 3 seconds, off 8.
Over 10 degrees it's on permanently. This way I get maximum absorption without the complexity of a PWM controlled pump.