What is the light where you read the test result? Daylight is best - easy at this time of year
- and it is known that fluorescent lights, including compact fluorescent energy saving bulbs, make the test tube look greener than it really is.
It is also known that people's eyes see colours differently. Some people never see the yellow zero colour even on tanks that have been running for years.
Ammonia in tap water is usually caused by chloramine, and prime will detoxify it for 24 hours, then it will turn back into undetoxified ammonia. Normally, the ammonia from chloramine has been 'eaten' by the filter bacteria within 24 hours.
Do you have any live plants in the tank? Plants prefer ammonia as fertiliser so they will remove a lot of the ammonia made by the fish. Floating plants are particularly good for this as they are very close to the tank lights, and can take in carbon dioxide from the air - these are the other two things needed for plant growth.
If you tell us the size of the tank we can recommend suitable plant species.
Stingray filters do have a couple of problems.
The first one to be aware of is that small fish can get stuck in the gap between the filter and the tank. The solution is to put a layer of filter wool in the gap so the fish can't get into it.
The second problem is the media. Part of the media is carbon-zeolite cartridges. Carbon is not needed on a routine basis and it will remove medication should you ever have sick fish. Zeolite removes ammonia and medication. The trouble is that zeolite gets full and stops absorbing ammonia; because it was removing ammonia not enough bacteria have grown so ammonia shoots up in the water. You are tied to replacing these cartridges before the zeolite gets full for ever.
The best thing you can do is take these cartridges out and use them to cut a piece of sponge to the same size and shape and use those in place of the cartridges. Any brand of filter sponge will do as long as it is big enough.
For anyone unfamiliar with the filter, here is the manual
https://uk.hagen.com/File/c02555ca-c56a-457c-80fc-f612958bbb2fOr get a better filter
And you should turn the heater and filter off when you do a water change. Heaters have been known to explode if they turn on when out of water, and filter pumps burn out if run in air. Water changes should be 50% a week when you have fish so the level will drop well below the heater and filter.
The stingray's pump is at the top of the filter so as the water level drops past the top of the filter the pump is running in air.
Fishless cycles take longer than 2 weeks. When I did one, it took 7 weeks.
Do you have any fish left in the tank now? If you don't, I suggest you buy a bottle of ammonia and follow the method in fcmf's link. If you do have fish, the best way to protect them is with live plants. We can help you with those