If it's just in the tank(s), I would disconnect them from your system and then give them a good scouring/flushing, assuming there is an access port. Drain them into a bucket until they are clear.
If you used 8 oz. per 50 gallon tank, then likely the water
is disinfected, but the scum you see is the product of the (formerly) living bacteria. The issue is getting it out of the tank.
Going forward, you should treat your tanks once a year, with chlorine, 1/3 cup per 100 gallons. Ideally, you should then test for residual chlorine (buy a pool test kit at HD which will test down to .5 ppm, which is adequate) ...if there is still residual chlorine, then disinfecting is complete. If not, add more chlorine incrementally until you get residual chlorine in the test.
A bigger question might be what shape the rest of your system is in. After the tanks are clean and you've treated the water while filling them, run the water through the entire system, let it "soak" for at least 30 minutes, and then test what's coming out of your plumbing
. If there is no residual chlorine, then there is more disinfecting required. Continue to cycle water through the plumbing
runs until you get residual chlorine in your test.
I would shy away from Water Shock from WM for the simple fact that you don't know what's in it and how effective it is at disinfecting. Bleach (chlorine) is the de facto preferred method, tried and true, along with UV and ozone treatment.