Hi beb,
Since your guy is still in there. You need to do daily water changes of approximately 50%. Make sure you use water conditioner in your new water. The changing of the water will help take out the
ammonia. Also its possible that high levels of ammonia can eat away at their fins. Its very toxic for them. When you're changing water try to use the same temp as your tank water, temperature changes can stress fish.
I hope one of the moderators sees your post because there's an additive you can add to your tank that will help neutralize the affects of the ammonia. I just can't remember what its called. Try a new post asking questions about setting up your tank uncycled. Don't feel bad you really meant well, its just important to read read read. Also keep asking questions. People on Fishlore are very helpful.
I hope you save your last little guy.
Its a pain in the neck, but really important that you do those water changes until your tank has cycled. Oh, did I mention you need a very good
test kit, not the one with strips.
API master test kit. This will help you to know when your cycle has begun, (I think you may be in the Ammonia and Nitrogen phase. Eventually ammonia will drop, then
nitrite. Read up on this stuff as I can't explain it as well as others. It could take several weeks. . .
Hope I was of some help?
Best,
Jess