Have you got any H2O2 or commonly known as hydrogen peroxide ?

Safer than bleach, put it in your tank. Run for 24hrs then drain and refill.
Over the course of 24hrs the H2O2 will break down completely ready for your next crop.

Your local hydro store should sell it ? Instructions on the bottle as to amounts to use. At 35% concentration I think it is about 50ml per 10 litres of water for sterilisation...

You can also use it during your grow so as to introduce more oxygen to the roots. This will be at a lower dilution ratio though.

Hope this will help