Why dilution is the emergency tool
Your filter bacteria will eventually grow to meet the waste load, but eventually is measured in days and ammonia harms fish in hours, burning gill tissue and suppressing the immune system while it circulates. The only intervention that lowers ammonia immediately is removing the water that holds it. Everything else in this response supports that central move or stops the level climbing back while the bacteria recover.
So do not start by dosing and hoping. Start with water, in volume, now.
The water changes, done properly
Change a substantial share of the tank, around half, in one go, using water treated with conditioner and matched to the tank's temperature so you do not trade a chemical emergency for a shock. Siphon from the gravel as you drain, pulling up the trapped waste and old food that are fuelling the problem. Refill slowly, then test again after the new water has circulated.
If ammonia is still clearly present, repeat with further partial changes over the following hours rather than one total change, which would swing temperature and water chemistry too violently. The target is a reading at or very near zero, held there.
Stop the inflow while you bail
Feeding is ammonia in waiting, so stop it completely for a day or two; healthy fish tolerate the pause easily and every missed meal is waste that never enters the water. Remove any dead plant leaves, snails or, worst of all, a dead fish, since decay is an ammonia factory. Rinse nothing in the filter with tap water and do not replace media mid-crisis, because the bacteria clinging to it are the long-term fix.
If you keep a second, healthy tank, moving a portion of its mature media into this filter is the fastest genuine shortcut available. Boost aeration and surface movement at the same time, because ammonia-damaged gills extract oxygen poorly.
Finding the cause while the readings fall
An emergency reading always has an author: a new fish added to an immature filter, a feeding increase, a power cut that stalled the filter, a dead fish missed behind decor, a medication that knocked the bacteria back. Identifying it matters because the level will simply climb again tonight if the cause is still in the tank. Test daily for the following week even after zero returns, and resume feeding at half rations, watching the test rather than the calendar.
If fish are gasping, clamped or dying despite falling readings, the damage may already be done internally; an aquatic vet or experienced fish shop is worth contacting rather than assuming the worst is over.
Writing each reading and change into the tank's Pet Passport on KinsLe leaves you a clear record of how high it went and how fast it fell, which is exactly what you need if the spike ever repeats.