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whats causing my ammonia to spike like this? im assuming itll be at 1ppm tomorrow and ill need to do a whole tank flush again!
This is normal. You still don't have enough of the bacteria required to convert the Ammonia to Nitrite.
Fish naturally expel Ammonia in their waste. Decaying, uneaten food is converted to Ammonia.
You are treating your tap water with a conditioner when you do a water change, right? Just checking you're not inadvertantly killing any bacteria you're cultivating.
Unfortunately growing the Ammonia-eating bacteria is the fastest part of the process. It takes a fair bit longer to get the Nitrite-eating bacteria to grow.
I'm afraid, for a good few weeks you're probably going to need to do large water changes daily or every other day.
Something else we haven't mentioned before - do you know anyone locally with an established aquarium?
If you can take a small peice of their filter medium, some foam, borrow an ornament, or even a small handful of gravel (put it in a small bag made from the foot from a pair of ladies tights).
All of these things will carry an amount of the necessary bacteria which, if placed in your tank, would seed your tank with the bacteria and speed up the process.
If you transport anything from the above list, make sure you keep it wet in water from the donor tank before adding to your aquarium.
Of course, you do risk transferring disease, if their tank is carrying anything, so only do this if you trust the fish in the donor tank are healthy.
i tested my tap water for nitrate and it come back 0!
That's very unusual. I find it hard to believe there's 0 Nitrate in your tap water.
I'm not familiar with a kit that uses powder. Mine needs 10 drops of two different compounds, one of which must be shaken vigorously for 30 seconds before adding to 5ml of the subject water. Then the mix needs shaking for 60 seconds!
Lots to get wrong!
my fish seem not as happy today, all the guppies are nipping at each other, and 1 of them is swimming in a corner....
Unfortunately, fish-in cycling is very stressful for your fish (and for the Fishkeeper!) and, it has to be said, deaths are almost an inevitability.

Thise that still practice fish-in cycling tend to use a cheap, hardy species before adding the remaining livestock.
Good luck with it all.