Your turtle’s filter needs to stay on at all times. Your turtle is always inside its tank, and it doesn’t stop producing waste or swimming in the water for long periods. Turning your filter off for a while will disrupt the good bacteria and the filtration process’s overall flow. By doing so, the water can get dirty and, sometimes, filtered water will backwash into the tank that’s already been cleaned. Even at night, your turtle may not necessarily be sleeping. They could go for a midnight swim and, when you wake up in the morning, the tank may be visibly dirty.
If the power goes out at your house and the filter shuts off for a few minutes or even for a couple of hours, your turtle will be okay. If it happens once in a while and you’re able to keep up with the water changes, that won’t be much of a problem. However, if it’s constant and you turn your filter off at night, which is seven or eight hours, then you’d be depriving your turtle of some of the necessary bacteria they need through the filtration system. They would be fine for a little while, but it would soon catch up with them and make them sick.