Yep, that indicates no obvious source for ammonia unless it was directly added by you in some source. If your rock is 100% not cycled and that sand was dry sand non cycled some fish could make real .25
Surely you cycled it with testing?
Details revealed: a fish not gasping at the surface where true ammonia poisoning would attack the gills.
You don't have to heat up change water with a tank heater. I could produce 15 gallons for a water change at 68 degrees and have it 79 degrees in about 10 mins by making water in blue container water bottles and putting them in hot bath tub to bring up temps.
The test readings at those levels do not help and don't factor so we are left not knowing the culprit, you may do large water changes at any time however, pour back in matched water slowly to not kick up the sandbed.
Second detail
The rock isn't coralline or living animals spotted, so it's relatively new cycled rock I'd assume. What did you use to discern the cycle you did was complete
Did you ever do a 4 ppm digestion test before adding animals?