If you do a 70% water change with the shop bought water, the pH and GH will change rapidly and probably kill the guppies.
Guppies need a GH around 200ppm and a pH above 7.0. The shop water is not suitable for them, but your current tap water is fine for them. If you want to keep guppies, do not use shop bought water unless you add minerals to increase the GH and pH. <- will the aquarium salt idea fix this
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Pictus catfish cannot be kept with guppies because they eat guppies. Pictus catfish need to be kept in groups and can grow to 8 inches in length. They need a tank that is at least 4 foot long.
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You can keep water in containers for weeks without any problems. The only time there will be a problem is if a contaminant that gets into the water. Other than that, you can have buckets of water sitting around for weeks or even months and it will be fine.