If it's not tapwater,Or faulty test readings,Then I might try 50 percent water changes while vaccuming the gravel or sand. Sounds like possible decay from fish waste,fish food,dead plant matter contrbuting,causing,ammonia level to increase. Maybe not enough healthy plant mass left to help with consuming ammonia.
I might wonder if the old clogged filter cartidge,sponge,pad,etc was thrown away, or cleaned under tapwater containing chlorine/chloramines.
If filter material was saved,cleaned in old aquarium water or dechlorinated water,,then two fifty percent water changes a week should knock ammonia levels down quickly considering present occupant's of the tank.
Would NOT add any more fish and would feed the fish I had ,, once every other day until ammonia levels and tank have stabilized.