Tub in master bath was slow draining last night and then this morning toliets in both baths backed up. Dirty water in both tubs now. No problem in other sinks in house. We don't have garbage disposal or dishwasher. House uses airstream septic system....about 6 yrs old. Pumped out 2 yrs ago. Help!!!!!!!!!!!!

Sulfuric acid... it's the best. Make sure there is plenty ventilation.

Hold off on the chemicals for now and lets try to unclog the drain with a snake. When you have standing water in drains chemicals will not reach your clog. Your sink drains are higher than the toilet and the tub so they may also be clogged but drain to the tub. Sounds like your main drain is clogged. Go to your clean out and carefully open it. Standing water means the clog is still further downstream so you rod toward it. If the clean out is dry the clog is further inisde the house so rod in that direction. While you have the rod available rod down the vent stack from the roof. Use enough rod to reach roof to basement floor plus 10-15' more.