Aeration is anything that creates bubbling or splashing on the pool surface --- having your return jet pointed upwards, a fountain, or a bunch of kids playing in the pool.

Regarding the HTH 3 in 1 tabs --- I am not familiar with them as I only use bleach in my pool, but I would suspect they are trichlor. Does it say they are stabilized? If so, that means that they do have cya in them. Some of those trichlor tabs have copper in them and if they do, you do NOT want to use them. You don't want copper in your pool.

Since you are filling from a well you may want to be sure that you don't have metals in your water. That can be an issue with pools filled with well water.