So, while I'm a newbie, I will tell you my experience. I grew with all of Advanced Nutrients Beneficials (Tarantual, Piranha and Voodoo Juice), plus Mammoth P and Hydro guard. The water never gets cloudy. Now, it may just be the AN line. But then again, that was my 1st grow and it was good.
I'm vegging my 2nd grow and am facing almost the exact same problem you are. 2 days later, have to wash in H2O2 29%. I even stopped using Bennies and to go sterile. But again, I'm just a 1 DWC grower. didn't help. After 4 washes, my lady was looking not happy and wilting.
The thing I noticed about whatever is in that water, is that almost over night, the ppm will stay the same, water levels will stay the same, but pH will go down a little bit first. like from 5.8 to to 5.6. Then after the 2nd night, it will be at 4.7. Did a lot of research and it says that cynobacteria among other bad stuff, will drop pH like crazy. this is before even slime sets in. I'm understanding that cynobacteria can live in cool water with or without O2. And it's a pre-cursor to pythium.
Someone can correct me if I'm wrong, it's just stuff I think I know. There's so much crap out in the internet these days it's hard to find good advice.
After running store bought beneficials, then trying to go sterile and after the 4th change, I decided I will never be able to run sterile just because of the conditions I'm in. It's in an apartment, I have a dog.... etc..... So I started brewing Richy Rich Tea....... (I think a lot of people know it as Heisenberg Tea but apparently Richy Rich was the originator of the recipe.). Anyway, 1 gallon of RO Water + 2 teaspoons of MycoGrow soluble, a dash of orka, a handful of Ancient Forest and 6 mL carbo load. Supposed to brew for 48 hours, but since time is the essence, I did a res change at 24 hours and used 1 cup (strained) tea after changing the reservoir, pouring from the top cuz I probably killed everything using the H2O2 (except the bad stuff).
This morning, pH only went down 0.1 and ppm went down alot and water levels just went down a little. with tea, it's cloudy. But not with the store bough bennies. My girl was starving looks like. So, tonight I will finally have my 48 hour brew and I plan to strain it well, pour in another cup and hope for the best.