Hello and welcome to FF Christiane! Adding a medication to an aquarium while another medication is still on water might cause problems due to interaction. You should always do a large water change and run well rinsed carbon in filter for 24 hours; then after removing carbon add new medicine. The floating problem could be due to a bacteria or something else. What made you think it was bacteria and treat for it? Also, be aware that some medications will affect your bio-filter and could cause nitrites to rise.

Read you are using Amquel with Cycle. That can cause problems too. The problem with Amquel and other similar products is that they are too aggressive removing ammonia and nitrites. With none of these compounds to feed the bacteria in tank (existing there or added by Cycle) they will die and cause the bio-filter to crash.

My recommendation is to do a at least a 50% water change with dechlorinated tap and then add Cycle per label instructions for a new tank. It will take a couple weeks for your bio-filter to recover if you do not add more Amquel. BTW, while using Amquel, some test kits will show incorrect levels of Ammonia and Nitrites. Good luck !