You want to have the pump on when adding acid so that it disperses throughout the pool. You don't necessarily have to turn it off after that, but definitely have it running when you add the acid and for at least an hour or two afterward.

The test kits Ben mentioned are the same kit, except that the "C" version has larger quantities of reagents in them. HOwever, there are multiple places you can order reagent refills if you need them.

Sounds to me like your pump is still too big for your filter, but I'll let the equipment experts comment on that. I have a 1 HP pump on my 29K gallon pool with a 300# filter.

Happy Memorial Day...early!!

Janet