What kind of granular shock did you use? Cal hypo or dichlor?

The reason it matters is that cal hypo adds calcium, and if your calcium is ALREADY high, adding more can cause it to 'drop out'. But, it usually doesn't 'drop' very well; the fine dust floats around your pool making it look milky. Calcium never needs (almost never? ) to be ADDED to a liner pool, but it can be too HIGH.

If you had algae, then the problem with the filter can be an issue, since the tendency is to keep circulating the dead algae, blowing it through the filter, and then circulating it some more. Do you have a way throttle flow back? (A valve, usually)

How are you testing?
The only kits that let you *measure* 8 ppm are DPD-FAS kits, but those usually include calcium tests, and you didn't include calcium readings.

Best wishes,

PoolDoc

PS Can you take pictures of your pump, your filter and them both together?

--This has nothing to do with helping you; only with helping others avoid the problem. But I'd love to have some pictures of a 22" filter with a 1.5 HP pump to use in a page on PoolSolutions in sort of a "don't ever buy this on your AG pool" caution. Those big pumps with small sand filters REALLY complicate the process of cleaning up AG pools, and there's no reason for it. If you can do so, great. No hurry, but if you do, email them to poolforum AT gmail DOT com