the large particles enable detritus like uneaten food and dead hermits etc plus algae spors and other things to settle between the particles, after a while it will raise the nitrate levels in the tank, for a fish only and if vaccuumed regually it would be ok as it does aid the ph, but for a reef its a disaster waiting to happen as corals and sensitive inverts like shrimps wont tolerate high levels, you will also get lots of algae and the look of the tank will be horrible, best to avoid it if you can and go for a decent sand bed