Hi
play sand is fine but make sure you wash it thoroughly or it will make your water cloudy, you can get aquarium sand but NOT marine sand as this often contains salts.
pH 6.6 is a little on the low side but still falls within acceptable limits.
May I ask how long you have had your Axie and tank set up, if it's very new with water readings all on 0 it may not have started cycling yet.
Welcome to the forum by the way
Ali