We have a 55 gallon tank that we've had set up for about 6 months now. We have 6 fish in it and all was good up until about a month ago when every time we'd take ammonia readings they were off the chart. We used to get slight ammonia readings but a 1/2 water change on a weekly basis would fix it. We have new media and sponges and everything in the filter and do 1/2 water changes, but nothing makes the ammonia go down. We put Prime and Stability in to change the toxic ammonia to non-toxic, but there's never any change in the reading. It is the highest that the chart reads at 8ppm. If this were true, the fish would be dead in a matter of hours. I've thought maybe it was giving a false reading, but when I measure the ammonia in our 20 gallon tank it's at 0.
What are we doing wrong and why did this all of a sudden start up after months of it being fine?