It's not that coral sand is bad for pH, but he fact that depending on your tap water it has the potential to alter it. If you have soft water then yes, you may well notice the effects of it but if you have hard water then it's unlikley that you'll see much change in the pH.
Silica is often a major ingrediant in most of the playsands I see around here.
I would not mix playsand in with anything for plant growth. The grains are small and will compact over a short amount of time not only slowing down any plant growth but also creating anerobic pockets. A more course sand would be a better choice.
For plants proper substrate and lighting are only two parts to the puzzle. You will also most likley need some form of Co2 injection and fert's.