Not sure if this was already suggested ... stabilized chlorine instead of unstabilized contained in bleach. Comes in tablet or liquid form ... I prefer liquid (3R Purogene for example) as I can measure any quantity necessary to purify any volume of water.
My new to me boat came with slimy water tank. That slime and the associated unpleasant aura was gone after the first treatment and "rinse". Now, I treat the water regularly and never empty the tank for winter storage ... the tank is spanking clean, and water taste great, always.
Here is a link to product info (UK) ... Purogene | Tristel
Here is a link to product info (US) ... Purogene Fresh Water Treatment Information
Available via lesser known online retailers ... do the search for your country/region.
Two of the USA online vendors:
Cademaster/BillyDump Online Store (Powered by CubeCart)
BTW, after using Purogene for 5 years I feel like this is the best kept secret. Why bother with bleach and chlorine that stinks, burns, taste awful, and evaporates after a while?