From what we've been able to tell this season, the Quikrete sand is very marginal in quality, and probably does not meet specs.

Definitely, do the DE test.

But also, go ahead and purchase a bag of labeled pool sand from a pool store. Then pop the top on your filter and grab a cup of sand from the filter. Replace the top, allow the sand to dry and then compare. Filter sand quality has at least 4 factors:
1. Range of sizes, within tight limits -- no very fine particles; no large ones.
2. Size -- average size is quite small.
3. Sand grains are sharp edged, not round.
4. Material is quartz, not other minerals.

If you can't tell by eye, take a dark sheet of paper, and spread some of the pool filter sand out in a very thin layer, and then photograph it in sunshine, using the macro setting on your camera. Then, do the same with the sand out of your filter.

If the Quikrete you have is like some of the other that we've seen, you may need to remove and replace the top 8 - 10" to get good filtration.