Probably call a plumber...

Work done by inlet in 1 hour = $\dfrac{1}{8} - \dfrac{1}{12} = \dfrac {1}{24}$

Work done by the inlet in 1 min = $\dfrac{1}{24} \times \dfrac{1}{60} = \dfrac{1}{1440}$

Volume of $\dfrac{1}{1440}$ part = $6$ liters

Volume of whole = $1440 \times 6$ liters = $8640$ liters.

To find the volume of the tank, we must find the rate of water lost through the leak.

First of all we look at the ratio between the time taken with the inlet pipe and without the inlet pipe. ${\frac{8}{12} = \frac{2}{3}}$ Hence the time taken without the inlet pipe is two thirds of the time taken with the inlet pipe.

Since the inlet pipe adds more water, it reduces the net rate of water lost from the tank. The relationship between time taken for the tank to drain, and the net rate of water lost from the tank is a negative correlation because it take longer to drain when there is a lower rate of water loss. Hence, we can say that the net rate of water loss with the inlet tank is ${\frac{2}{3}}$ of the net rate of water lost without the inlet tank. This means the 1/3 less of rate of water is the inlet tank.

The inlet tank is 6 litres per minute, so we can make the ratio: ${6 : \frac{1}{3}}$ Therefore, ${18 : \frac{3}{3}}$ And so the rate of water lost without the inlet tank is 18 litres per minute, in other words, the rate of water lost from the leak is 18 litres per minute.

It took 8 hours for the leak to drain the tank empty, so if we find use the rate of water lost of the leak, and find how much water is lost in 8 hours, that is the volume of the tank.

18 litres per minute. ${18 \times 60}$ 1080 litres per hour. ${1080 \times 8}$ 8640 litres per 8 hours. The cistern holds 8640 litres .

Sorry if I used too many words.