You've been asked what's really in molly, but do you know what's really in fish and water? It turns out it could be molly. But not just that, ketamine and caffeine too. The Washington Post pointed to a study that found "big spikes in drugs like ecstasy, ketamine, and caffeine" during the Spring Scream festival in Taiwan.
The article doesn't mention any US or EU festivals, or actually any other festivals at all, but it certainly stands within reason. The Washington Post pointed out that various other "one-off events" and even weekends can cause a surge in this contamination, citing a couple other examples, and noted that pharmaceuticals are another known, general water contaminant.
Fish and water have long been sources of contamination from runoff. The most famous example is probably the film Erin Brockovich, in which Julia Roberts investigates why so many young locals are getting cancer and dying. Fish, in particular, has also long been associated with high concentrations of mercury.
We wonder what the levels of these "illicit drugs" were relative to other, unfortunately standard, contaminants. It's not clear what the study that the article cites is suggesting. The article does note that the long-term effects of this kind of contamination isn't known, but it's unclear how high these levels actually were. It seems to be more of a statement that it has happened, and not what dangers it actually poses.
While we would never suggest or advise that you do drugs at all, this certainly seems like an awfully hard problem to eliminate, though contamination of any variety just isn't good.