Its also worth pointing out that if you have slime in the water barrel, it is likely to be forming elsewhere in the water system. So its worth mixing up a water barrel full of Milton solution and pumping some through the water systems, (removing the water filter first if you have one and replacing it with a an empty or blank one to avoid damaging the filter)
Check the instructions for the filter about replacement intervals.
Always follow the dilution instructions for the sterilising fluid, and don't leave it in the system for longer than the recommended period. Always flush out with fresh water.