Welcome to TFP! Here we recommend either the TF-100 (link below in my signature) or Taylor K-2006C. The TF-100 is a very good value. Many users also like the magnetic speed/smart stir tool for easy mixing. As for the PoolMath APP, you can use it for free (without saving results) or subscribe to maintain testing logs and other history. A great tool to help with dosage amounts and overall chemistry.
Welcome to TFP! For a wealth of pool care information please check out our free Pool School. If you are in need of specific help then we encourage you to Register and ask us here in the forum!