I remember I had some and I stopped feeding them when I got bored of them and they never died
You can't really clean their water but you can move them to a bowl or something and then clean their tank.
If I remember right actually, you can buy a water purifier packet from the makers and maybe that can clean the water?
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As soon as I saw this thread I thought of Lex. Her username is Sea Monkey.
I did this too.
Anyway. I had some when I was younger, and we bought them in a kind of pack, which came with everything we needed (tank, water purifier, etc). I think the water purifier was the closest thing we ever got to cleaning the tank, since a) it never got that dirty and b) they never lived that long anyway.