Hi Lisa,
The first option: Remove the sand.
As long as you have that percentage of coral sand, it will constantly leak out carbonates and raise your pH.
Adding more play sand won't really solve the problem because this doesn't dilute the concentration of carbonates in the coral sand. The carbonates will still leak out and raise your pH.
I'm afraid you have to put in the elbow grease. I wouldn't reccommend adding the new sand directly into the tank because the particles will become suspended in the water and create a huge mess. You can test the outcome for this by adding a handful of sand to a bucket of water.
You could try this approach:
1. Remove all the tank water and place into buckets
2. Remove the sand
3. Add new sand that has been washed thoroughly and is free of carbonates (in particular magnesium and calcium carbonate)
4. Create a circular dome (dome should not contain sand) at the base of the tank, and pour/siphon the buckets of water into the dome. This should reduce any mess.
Or alternatively, you could wait to see if your pH stabilises. That is, you can wait to see if the amount of organic waste ("bioload") balances things out and prevents your pH from increasing.
But that's all I can think of, so yes best of luck with it all!
Jay.