Wipes — even the "flushable" ones. This one gets people every time. The packaging says flushable, so it must be fine, right? Not really. "Flushable" just means they'll make it past the toilet. What they won't do is break down in your tank the way toilet paper does. They accumulate, form clogs, and can eventually block the outlet pipe that feeds your drain field. Regular baby wipes, makeup wipes, and cleaning wipes are even worse. None of them belong in a septic system.
Grease and cooking oil. Pouring bacon fat down the kitchen drain feels harmless when it's liquid, but it cools and solidifies inside your pipes and tank. Over time it creates a thick layer of scum that the bacteria can't break through efficiently. Pour grease into an old can or jar and throw it in the trash. Every time.
