I agree with the comments above. Bacteria supplements can deal with the ammonia even if the bacteria is not in the filter. Stop the supplements and the tank crashes. I don't hold with adding bottled bacteria myself, seeding the actual filter with media from an established tank is the surest way of getting the bacteria where you need it - in the filter.
A ph crash is common when cycling especially if your Kh is too low. You didn't say what the Ph dropped to. If it was 6.5 or lower then this itself will hinder the development of beneficial bacteria. If the Kh is too low, below 5 deg, then the water hasn't the buffering capability to avoid an acid crash. Have you tested your Kh?
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