Bleach, really? Would you drink it? Bleach kills all oxygen molecules and sets the body, any body, up for disease. I wouldn't use bleach if you paid me.
No need to paint food grade barrel. I have two 11 gallon tanks that I use for my girls. When I fill them up, I add a teaspoon of food grade 35% H2O2 to each container. It sanitizes their water, and add extra oxygen which is good for them. Since H2O2 kills all bacteria, good and bad, even inside the chickens, the girls also get some good old homegrown kefir with its 42 beneficial yeasts and bacteria to replace what they lose through their water. Awesome stuff. No bacteria, no mold in the water tanks, no bad germs inside the chickens. Good for people, too.
bleach in a small dose is perfectly safe. As long as it is just bleach and there are no additives in it. It doesn't take much to do the job. I am getting ready to bleach out my well and all the lines up to the house. Has to be done every few years especially if you have had work done and the well cap off.
These solutions will reduce or eliminate slime and most disease organisms in water, drinkers, and water lines.
For Constant Use
1 teaspoon chlorine bleach (sodium hypochlorite) in 5 gallons of drinking water
This solution provides 11 ppm chlorine for sanitizing. The birds will drink the water and not be harmed by drinking it. They may need a short time to become accustomed to this solution. A more dilute solution with half the above level of bleach can be offered for a few days before using the 11 ppm solution. Clean the waterers thoroughly each day to get the best effect.
Weekly Sanitizing Rinse Solution
1 oz Chlorine Bleach in 6-8 gallons water
Rinse, soak, or expose equipment to this solution. Let stand at least one hour, then rinse with fresh water. This solution contains equivalent to 45 ppm chlorine. The procedure is most effective if conducted on a weekly basis. Remember, chlorine disinfectants are inactivated by organic matter. Clean all equipment well before using chlorine rinse solutions."
I would bleach if its a container that had organic material like rootbeer syrup or if it had an off smell. I would bleach if I thought animals had something contagious I might even throw the waterer away. But for the most part I use organic soap and a toilet brush designated for that purpose.
For what its worth The blue barrel will be just fine its opaque enough to not grow algae. Algae in of itself is benign. Its just a symptom that there is enough organic material for it to grow in the water.
I have a 110 gallon horse water tank and an 80 gallon Goat water tank. I use Gold fish in them to keep the insect population down graze on the algae on the walls and to eat extraneous organic matter. The goat water stays perfectly clear... My horse is a mouth washer so I have to dump hers out and lightly scrub about every month or two. I pull the fish out beforehand and scrub the tanks out with sand. Water is precious in my area so I let her drink down to half a tank.
I take it you don't drink city water. and for those who don't know what H2O2 is its Peroxide. Never thought of using that but then I don't use chemicals much at all. Rock salt for scrubbing and 105-110 degree days filled with natural UV for the empty dry containers. UV is a natural germ killer.
The well thing.... has to be done. My water coming out of the faucet is smelling like rotten eggs.