Lots of oxygen in the water does not stop ammonia from killing fish. High oxygen levels do help the beneficial bacteria to grow faster. It also gives the fish more oxygen to live on assuming the levels were low to begin with. But in an average aquarium it doesn't make much difference. Water can only hold so much oxygen and all the aeration in the world will only put a certain amount into the water.
Getting some mature media from their tanks might help but if it was out of the tank for more than an hour or two then it is probably not going to do much.
The beneficial bacteria is unable to process ammonia at extremely high levels. You might find because the level is 8 or there abouts, the bacteria won't do anything about it. Normally for filter development 5ppm is as high as you want it to get. Quite often when the level gets higher the cycling process stalls. If you have fish in the tank then you want to keep the ammonia as low as possible.
Bacterial supplements can help get things going a bit quicker.
What filter materials do you have in the filter?
Is there any small white granulated substance called ammonia remover in the filter? If so remove it because it will hinder the growth of the filter bacteria.
Ceramic beads, noodles, foam blocks and filter pads should all be fine to use.
Chances are the 3 zebra fish you lost when you changed the tank from brackish to freshwater were killed by osmotic shock. Basically the fish absorb salt from the brackish water and when you dumped them into freshwater all the salts leached out of their bodies and they filled up with freshwater and drowned.
You would have been better off doing 10% daily water changes for a couple of weeks and that would have diluted the salt slowly enough for the fish and filter bacteria to adapt to the freshwater. Then once the salt levels were gone you could start doing bigger water changes.
Try to keep the ammonia levels below 5ppm and make sure the filters are run continuously. Monitor them daily and see how the level goes. It should not take more than about 2 weeks for the ammonia eating bacteria to develop in a tropical tank. A couple more weeks and the nitrite eating bacteria should be sufficiently developed as well.
Edited by Colin_T, 03 March 2008 - 05:13 AM.