One small concern Harry, how deep is the layer of sand?
When the sand is deeper than an inch, water flow inside the layer becomes inexistent, and bacteria quickly use all the oxygen. In time, this causes anaerobic pockets to appear, and then anaerobic bacteria con grow on them, decomposing waste and creating dangerous gases. This can really affect the quality of the water and cause severe problems if not controlled.
Using a layer no deeper than an inch is one way to avoid this. Alternatively you can move around all the sand periodically, but i´m afraid it would cause clouding of the water.
Consider this, since a little change can spare you quite a bit of trouble.