I have always left algae tablets in overnight, they are always gone by morning. I make sure to put them in "last thing" before turning off the lights for the night so the other fish don't avail themselves of them. I assume my pleco gets them (as he has developed a very robust buddha belly) but I have seen my Clown Loaches go for them a bit as well. I never put in more than 1 or 2 an evening, typically 3 nights a week.
I've never noticed any water fouling, but I have a 55 gallon with an abundance of filtering.