> the couple carried 6 quarts of water apiece.

So two containers, and with half their water gone, they could have transferred the rest to a single container and filled up the empty one with tadpoles. Then they could discard it if they found a better source before they needed to drink it.

I agree fauna in the water don't mean it is safe (Terry Pratchett used to wonder if newts got out to go to the lavatory), but even dirty water may keep you going long enough to get to medical care before illness kicks in.