Childrens play sand is an ideal sand substrate as it has been sterilised or sand from an aquarium (just ensure it is for fresh water aquariums not tropical). Do NOT use sand from the beach.
Sand will need to be rinsed thoroughly before adding to tank. When I put sand in my tanks, I used the 'bucket' method - put sand in bucket, pour in water, give a rough stir with hand, pour out water - continue procedure until water run off is clear. When clear, put sand in tank.
With Ammonia/Nitrite/Nitrate readings all at zero, your tank hasnt cycled. You will need to monitor the water daily with test kits and do partial water changes of 10-20% daily. Ammonia or nitrite readings of higher than zero is toxic to an axie. Cycling can take from a month to a few months to complete.
This article explains cycling http://www.caudata.org/cc/articles/cyclingEDK.shtml
If you have any queries, please give us ahoy - cycling can be difficult to grasp.
How large is your tank?
Your tank temperture is lovely for an axie.
I note you are from Australia - Bunnings sells childrens play sand - this is far cheaper than an aquarium - I purchased 20kg of play sand for about $6 AUD.