i never use bleach, though i do scrub with a brush every Monday. Also, i add a sloosch of apple cider vinegar to the water tank periodically. It keeps down the algea. Also discourages insects and is good for the horses esp in the heat.
I knew a girl who put goldfish in her water trough to eat the algae because she just didn’t feel like cleaning it… it was so nasty to me, the water smelled terrible 
however someone posted earlier on this thread “pristine water is our invention” which actually made me pause a lil bit, like damn true!!! I feel like I’d feel safer with horses drinking water with little fish and tadpoles rather than random chemicals.
Not here calves will step into a bath tub made into a trough.
I dont know about tadpoles but it will probably work, as it does for mosquito larvae.
Put in cooking oil to put a layer over the surface, so the larvae can not come to the surface to breathe air.
The horse’s muzzle breaks the layer for the horse to drink and is not poisonous. The layer reforms when the horse leaves.
