Looks good mate.
So your tap water has about 5ppm nitrate so you’ll never read 0 even if you did a 90%+ water change.
For your current bioload you have a cycled filter.
Each time you add to the load (extra fish, turtle or stingray) you will see ammonia/nitrite become present again for a short period of time as your beneficial bacteria colony grows to consume the newly available food source. This is called a mini cycle and is normal. If your greatly increase the bioload in 1 hit this mini cycle can be detrimental. Take it easy, back of feeding, use Seachem Prime at detox dosing, do an extra water change and don’t forget salt to help fish deal w nitrite poisoning.
All the best mate