Go to one of you local spa dealers and they should be able to set you up. There are a lot of different care routines to pick from.
I use a Dichlor/MPS combination system (search: Vermonter Method). I use Dichor for maintenence dosing after each soak and alternate between Dichlor and MPS for shocking only as needed (I go by the smell). I also use the silver ion cartridge and an ozone system and have very little trouble with my water. I haven't had to use any other chemicals other than a calcium increaser due to softened water on a refill.
I would stay away from a Baqua system. It can eat some types of plastic and may have other issues if you do not follow the dosing instructions exactly. (I bring this up because you mentioned this was a used tub)
Hope this helps
Rich
