At the levels we're talking about, just leave the bleach in the tank. It's no stronger than the chlorine normally put in the water supply, you may not even smell it.

So how much you recommend?

I'm clueless in this area. I think it's 900 litre tank

At the levels we're talking about, just leave the bleach in the tank. It's no stronger than the chlorine normally put in the water supply, you may not even smell it.

So how much you recommend?

I'm clueless in this area. I think it's 900 litre tank

According to the EPA, 4 PPM: http://water.epa.gov/drink/contaminants/basicinformation/disinfectants.cfm

Which using this calculator: http://www.troublefreepool.com/calc.html equates to 58ml of household bleach (with 6% chlorine) in your 900L tank.

If the tank has any air flow, the chlorine will evaporate quite quickly on its own.