Why is lots of water bad for septic, and how can I take long showers without hurting it?
It is similar to changing the water on a tank of fish, not quite but the best comparison I think of on the spot. Your septic system works by natural decomposition aided by bacteria, surprising similar to your stomach. Putting that much water into a septic field at once can disrupt decomp process by "shocking" the system or flooding/shaking it and moving the bacteria away from their food source. There are companies and even apartment complexes using septic so it all depends on the size of your system and if/how much you feed you system bacteria (there are quite a few products for this). If you find yourself needing to pump out your septic too often, or at all depending on the system, then I'd look into supplementing it with some products.