So its possible that my tank was cycled then before I moved the hornwort? That would make me feel so much better!
I think that's probably wishful thinking, lorindaleigh. It's quite possible that your tank has never cycled completely. You have no idea what your real test results have been because you thought the test strips were accurate.. which they're not. It's probably come real close to making it through the latter stages of your full cycle several times, but then something happened that killed off the progress you'd made, which put you right back to an earlier stage again... without even knowing it. None of the test reading results you taken until getting that new liquid test kit were reliable. So... when you thought it was safe enough, you've added a full stock of fish now that's producing a bioload on a continual basis that only a fully cycled tank can process. That alone will through the balance off.
If your tank was truly cycled, then wiping out a tiny percentage of bacteria when you clean a filter, or vacuum the gravel, or remove a flaoting plant is not going to jack up your ammonia levels. A real cycled tank will take such minor setbacks like those in stride.. with no measurable ammonia peak whatsoever.
The real problem now is you need to let the cycle (or mini-cycle) actually finish its course in a tank that's stocked full of fish already. That's a very difficult way to do this... you can expect to see very unpredictable test results because so much is off balance (but this time, they'll be accurate)... and it's going to take an real long time for everything to stablize.
Let the test kit results tell you now based on actual ammonia, nitrite, and nitrate readings when a water change becomes necessary to protect your livestock. You'll be changing a lot of water because of all those fish, believe me. Yes, you are overfeeding. And what's worse is that food is being left to decay and continue to foul this water (adding even more to the bioload) for a real long time. You don't need to throw in some food and let it sit there for 8-12 hours. Do some target feeding and anything left over that they didn't eat within 15-20 minutes gets removed then. Don't stop rinsing the goop from your filter or gravel vaccing it up from under the substrate. The less excess waste that stays in the tank for long periods reduces all the gunk the bacteria needs to convert.