For us, we flush our tanks every time they get used. By that I mean, when we get home, we flush tanks right away while as I call it, stuff is still in motion. Really haven't had any build up of sort in the black tank. we do use chemicals for waste break down whenever we use the trailer. Even if it is only a day or two trip. My belief is that it helps form letting things build up. I also made my own wand out of 1/2 inch PVC pipe with a double 90* bend on it, put it down the toilet and let er rip. This is used only if I have been dry camping and had a little pile up of paper and poop. I also use the five gallon bucket method to flush tank. these things work well for me. Good Luck & Be Safe ! ! !