Actually, the kit we recommend is a FAS-DPD kit which is better than a DPD kit. The Taylor K-2006 or 2006C is the one we suggest and you can get it through the test kit page in my signature below.
You should only backwash the filter when the pressure rises 8-10psi over clean filter pressure.
What kind of shock did you use -- meaning ingredients. What is your CYA level and your calcium hardness level? Your OTO kit won't be able to test for those two.
If available at YOUR Walmart (check availability), get the HTH 6-way DROPS test kit, which is compatible with the Taylor K2006. Test the pool as soon and you can, and post the results. If you get the 6-way kit, ALSO test the water you FILL the pool with, especially if it's a well, and post THOSE results as well. (The HTH is the best available kit you're likely to find locally, but it's not the K-2006. It can only provide rough measurements chlorine levels above 5 ppm, and it measures "TOTAL" hardness, rather than "CALCIUM" hardness, which is not ideal.)
Also, please fill in your pool's info in our pool chart to make it easier for us to help you.
Pool Chart Entry Form
Pool Chart Results
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