Wipes of any kind including flushable ones, grease, coffee grounds, paint, solvents, and heavy bleach volumes. Regular toilet paper is fine. Additives are not necessary and are not a substitute for pumping.
More detail
A septic tank works by letting solids settle and bacteria digest them. Everything on the avoid list either does not break down, kills the bacteria that do the work, or fills the tank faster than the pumping schedule assumes. The worst offender by a distance is wipes. Flushable wipes are flushable in the sense that they leave the bowl, and they do not disperse in water the way toilet paper does. They accumulate in the tank and snag in outlet baffles and pumps. Paper towels, feminine products, dental floss, and cat litter behave the same way. Grease and cooking oil pour as liquid and solidify in the tank, thickening the scum layer and eventually reaching the outlet. Coffee grounds do not decompose meaningfully and go straight to the sludge layer. Chemicals are the quieter problem. Paint, solvents, pesticides, and large volumes of bleach or drain cleaner kill the bacterial population the system depends on. Normal household cleaning, including regular toilet bowl cleaner and a typical laundry bleach cycle, is not a problem at ordinary volumes. Dumping a gallon of bleach to clear a smell is. Toilet paper is fine, and there is no need to buy septic-specific brands. Any standard paper breaks down adequately. On additives: the products sold to restore or supercharge a tank do not remove the sludge layer, which is the thing pumping removes. There is no additive that substitutes for a pump-out, and the EPA position on this is unambiguous. Money spent there is better spent on the pumping interval. Garbage disposals deserve their own note: they work on septic, and they add roughly 50 percent more solids, which shortens the pumping interval to every two to three years. the mix of pre-war stock in the older Columbus neighborhoods, mid-century ranches in the inner suburbs, and large 1990s-onward subdivisions in the outer counties Households that moved from a sewered part of Greater Columbus and Central Ohio onto a septic property are the most common source of wipe damage here, simply because nobody ever told them. If you are new to septic, this list is the highest-value thing to hand to everyone in the house.