I mistakenly overstocked my 29 gal tank before the cyclying process was
finished. The tank is 7 weeks old tomorrow and I lost 2 platies this past
week due to ammonia poisoning. The ammonia level has been sky high
for the past couple of weeks.
The other 19 fish are doing well. I have cut down to feeding e/o day and
am doing small water changes. I also put 2 plastic plants full of algae from
my 10 gal goldfish tank (well established for months ), into the 29 gal.
My nitrates and nitrites have always read 0.
Has this 29 gal tank cycled ?

If the ammonia is 3ppm or higher, you should be doing at least 50% water changes. Small water changes don't make a big enough impact. You should see nitrites soon.