Water is a great conductor of heat, that is why it is terrible for thermal storage. in fast out fast. what you want is something like dense stone or hard concrete. I do not know what you have there but we have a variety of types of blocks and bricks. we have what we call cinder block which is light (for a block)  very pores and does not hold heat well. then there is concrete block (vibrated is better) it is dense and holds heat well. some red brick work well. the speed in which brick and block transfer heat depends on surface to mass (or the amount of webbing). stone like granite also works well and the surface are can be regulated by the size of the stone. if it were me I would use concrete block or hard stone.
 If your collector isn't big enough for your mass it will not heat up enough.
 you may think about a water soars heat pomp. when the water and mass has gotten to the point it isn't giving off heat be cos it has cool down (after several cloudy days) a water soars heat pump will continue to suck the heat out of the water/mass. till you draw the water down to almost freezing point if it is built correctly