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Yes your guppies can be in the tank during a water change.
It will be a lot easier to know what is going on if you have your own testing kit. Sounds like a strange fish store that doesn't test nitrAtes and won't tell you numbers! Without knowing your nitrite levels, I'd say do a 50% water change every day until nitrItes read zero. Nitrites are dangerous for fish, and it sounds like you might be in the nitrite spike phase of a cycle.
That's not good service, imo.
Anyway, you should have, if not a test kit, at least 5 in 1 test strips, nitrite strips and ammonia strips on hand anyway. Levels do need to be checked from time to time. You can get those 3 strip tests at Walmart, and most pet stores will also sell kits.
Dont forget to dose Prime it helps lower down amonia & nitrites. I had that exp before when my water was hazy and my fish was on top of water gasping for air. Good thing i invested in api test kit and found this forum. I did all the necessary things to keep my fish alive & healthy while my tank was cycling. Watch nitrogen cycle on youtube,i think i have watched all of the videos about nitrogen cycle while my tank is cycling lol.
Thanks guys I started adding prime yesterday how often should I add it. I have a master test kit coming Wednesday (found it $15 cheaper online than in store).
Read the directions on the bottle for dosing frequency. Just note that prime will detoxify the ammonia but it doesn't remove it so it will still show on your test until the bacteria can break it down.