Hello; I cannot say for sure what is on the zebra. From the picture it does not have the usual fuzzy cotton look of a fungal infection often called cotton mouth. It is not ick in my opinion. Most likely some sort of infection but I cannot narrow it down. From the additional description of the poo, I would lean toward a bacterial infection but this is a very weak guess.
That being said the quarantine tank(QT) has done it's job. With proper care the disease cannot get into your other tanks. Do not use any equipment that has been in the QT with any of your other tanks; nets, heaters, filters and such. Do not have your hands in the QT and then in the other tanks without a good washing. Whatever treatment you try in the QT may not work, and you may lose any or all fish in it. It may be best to allow any fish that survive to stay in the QT for an extended period of time to be sure they are disease free before placing them in other tanks.
I have gone to smaller numbers of fish at each purchase and to extended time in a QT. Parasites that can take four or more weeks to show up have been mentioned on posts in fish forums. This way I risk fewer fish at any one time. The down side is that the QT is occupied for a month or more and I cannot buy any new fish during that time. If I add a new fish the quarantine clock resets and the waiting period starts over.
Good luck. You may look thru the disease sections of fish forums or do some searches. Some forums have pictures that may help you narrow the search for the problem.
Often in my experience there is little to be done for diseased fish. After the disease has run its course it may be good practice to clean the QT and all the associated equipment before putting it back into use. Good luck.
