KISS.
Change water if ammonia registers on test kit. Do it whenever needed, as dictated by ammonia readings, not on some random fixed schedule.
Test for nitrIte - if it registers, change water and add three *teaspoons* of salt for that size tank to combat the nitrIte.
By all means put the plants back, any extra filtration will help.
Jumping up and down, going crazy and adding lbs of salt when your immediate problem is ammonia? Crazy.
If I had to bet in the absence of other info, wouldn't be surprised if the small-volume water change regime let the pH drop so the filter quit. pH may be up now, did you take a reading when you got the very first ammonia readings? Course it doesn't matter now, just fix the ammonia with water changes not adding chemicals.
As a secondary matter, check that the water is being spread evenly over your bio media - if not, it's possible some of the media isn't being used and you don't have as much effective media as you think.