You need to do 50% water changes until your nitrites get to .5 ppm.
This might mean twice a day until you the bacteria will catch up enough to keep stable at that level and lower itself.
25% water changes once a day won't cut it. You've dropped them to 3 but by tommorow they'll be back up to 4 or more. If you're worried about slowing the cycle down with water changes you should also know that to high of a nitrite level will also slow it down as the nitrifying bacteria are very fragile and those levels could be killing them off or keeping them from multiplying.
If you want to save the fish and the money you've spent so far you'll want to do the 50% water changes.
Strips are very inaccurate, the test tubes are better on the ammonia,pH and nitrite, not so much nitrates.
As for the plants..
How many watts is the light over the tank? What plants are in the tank?
If they're not compatible with the wpg you have then yes dying plants can contribute to the high nitrites in the form of ammonia. If I'm wrong someone will correct me.