With or without the test kit you will soon find out that having no filter on a fish tank will not work and that the water changes will not suffice to keep the ammonia and nitrite at zero ppm. It's just that with the test kit you may find this out in time to save the fish.
To keep the ammonia at zero ppm, you will need to do a 100% water change every 10 minutes to an hour depending on the bioload... as opposed to your current partial water change once in 2-3 days.
I too am afraid you will soon find that raising large pets such as RTC and TSN requires a substantial budget as there are bare unavoidable necessities that need to be met. If not met, it will result in sick and dead fish.
For example a filter on a fish tank.