Gravity water feeder

[Themogfather]

I brought some gravity water feeders to encourage my cats to drink more water. They look a bit like the water coolers you get in offices.
They have been working great and i have seen my cats drinking a lot more than usual.
Last night when i was feeding them i noticed the water in one was low and so decided to refill it. When i picked it up i noticed 2 large black things in the water reserve. When emptied out they were 2 live bettles.
Has anyone else experienced this and is it a danger to my cats?