Duall, you need an API freshwater test kit ASAP. You need to know for yourself what your ammonia, nitrites, and nitrates are. When a tank is cycling, it needs much more frequent monitoring so you know where you're at. You could lose fish if you don't know if you have high ammonia, nitrites or nitrates. I tested daily when cycling. Now that I have cycled tanks, I only test once or twice a month. Its about $25, but WELL worth it! I would be lost without mine. I don't want to depend on a pet store for occasional testing, that likely won't be very accurate, anyway. I want to know NOW, so I can fix the problem right away.
If the lady said your ammonia is high, then its probably nitrites that are also high, NOT nitrates. Ammonia goes up first, then nitrites, and finally, nitrates. When that happens, your tank is nearly cycled. But nitrates won't normally show up until toward the end, and at that point you should also have zero or nearly zero ammonia and nitrites.
Also, having live plants in the tank really helps.
If I were you, I'd be changing at least 30% of the tank water on a weekly basis. Not just 15%. Especially on a cycling tank. Even on my cycled ones, I do 30% each week.
Gouramis and sharks will often not get along, especially as they mature and get older. When they are babies, things can be a different story, and all seem to get along, then one day they are chasing, nipping and trying to kill each other. Angels can also be aggressive. Many people are surprised to hear that, but its the truth. Always research fish BEFORE buying so you know how big that fish will eventually get, and how aggressive they are, and what you can safely keep them with.
You were smart to change one filter cartridge and only rinse the other. Rinse in old tank water that you are changing out, NOT tap water. And never change both cartridges at once. I'm sure you already know this. You are probably having a bacterial bloom which sometimes happens in cycling tanks. It'll clear itself up in a few days, usually. Not usually a big concern.
I personally wouldn't use Ammo Lock. Just check your parameters frequently and do at least a 40% water change if any of your parameters are too high, like ammonia, nitrites or nitrates. That will solve your problem. You might find yourself having to do water changes every other day for a while, but that's cycling. After you are cycled, you won't have to worry about it again. I use Prime, as its cheaper than Stress Coat and lasts a lot longer. I use it for every water change, and even when topping off the water due to evaporation. Takes the chlorine out. I wouldn't use Stress Zyme if I were you. I don't think its necessary. Just Prime.
Most important though is the API test kit. You could so easily not know in time if one of your parameters is way too high, and that's when you lose fish. That could be what is happening with your gourami. If you have your own kit, you can test daily, or every other day (I did it daily when cycling) to make sure your fish are safe, instead of having the pet shop test it with strips, which are much less accurate. I wouldn't be without the test kit. They are lifesavers, literally! Go get one today if you can. You might still be able to save your gourami if you act fast. Keep us posted!