Hi,
Out of interest are these large drinkers?
Any body of standing water will tend to stagnate and go green in this warm weather, if using self-fill drinkers for pigs, keep size to a minimum, we use 2 1/2 and 3 Litre self fill drinkers, so the water does not hang round long enough to go stagnant.
As to "can they drink it?" - ask yourself the question, would you drink it?
Thanks