I wouldn't use the turtle's water. There's no point in it, as it has ammonia already. If I may speak for troi, I think he was talking about getting something from your turtle tank because he thought it was cycled. If it's not, then there is no point. On top of that, the bacteria are not free-swimming, so there won't be any in the water itself.
Also, when you vacuum the gravel do it very gently. Bacteria also colonise the gravel and disturbing the gravel too much will kill it. For the same reason don't mess too much with the filter at the moment -- that will kill off some of your bacteria.
You say the fish look fine. If their fins are erected, they don't dart around or are too lethargic (remember, cories are a bit shy and they don't swim a lot during the day even at the best of times) or behave strangely in any other way it means that you're on the right track.
I have an extensive experience with AmQuel and it really an excellent product, even in higher levels of ammonia. It should have by now detoxified all of the ammonia and nitrites in the water and your fish should be feeling better, so things should be done gently now. Change 10% of the water with new water. Vacuum gently, but not more than once a week. Unless your ammonia levels shoot up, there's no need for more AmQuel only dose with half the amount of AmQuel. Let nature take over from you, it will.
Two more points:
1. NitrAtes. These should not and probably never will be 0ppm. They are the end product of the nitrifying process and should not exceed a level of 40ppm (and some people claim 20ppm). Some of the nitrate will be used by the plants, if you have any, the rest will be dilluted when you do your weekly/bi-weekly water changes. Don't worry about it now (unless it's over 40, which it's probably not)
2. Have you used the ammo-carb? If not, I would suggest not to. They will filter the ammonia out of the water, but your tank will not cycle for the full production of ammonia of your fish, and when you do take it out later, your bacteria will have to adjust again, which means another mini-cycle. 2ppm of ammonia can easily be treated by AmQuel Plus which will make it safe for the fish on one hand, but available for the bacteria. If you have used it, don't take it out though. In about two weeks it will gradually stop working and I'd suggest taking it out in a month or so. But I would suggest using that ultra carbon you mentioned. That will filter the excess AmQuel out of your water, which I think is good, but not absolutely necessary.
I hope I'm not contradicting anything troi said

. If you have more questions, I'm around at the moment so don't hesitate.