And don't forget to test your ammonia after a water change as well.
My thoughts after reading these threads:
This quote is the key to all of this, and to the 30g as well, constant testing during your cycle is what matters, then you can figure out what to do based on your test numbers. the same with an estabilished tank. Many people get caught up in do's and don'ts, and in how long a cycle takes etc. There are many true facts stated in this thread, and a few slighly skewed by mis-conceptions. The gunk from the pet store filter, might have been enough to instant cycle the tank, This is one of the few plus's you have going with your current situation. the easy way to tell is to test constantly. Yes water changes slow the cycle time, but you have fish in there already, and their health through the cycle is most important. 6 weeks at very low ammonia levels is far less damaging IMO than 2 days at 4ppm. Since you aren't trying to hurry the cycle so you can get your fish, it really shoudn't matter how long it takes as long as your fish aren't subjected to spikes. I have cycled tanks in 1 week and sometimes it takes 6 weeks depending on bio-load and filter type. I have done fishy cycle's where there never was a significant spike, I know there wasn't because I tested twice a day throughout the cycle. With the gunk from the pet store filter in place, you essentially have your bacteria. The cyle is technically complete (assuming gunk means media of some kind)each type of bacteria that you need is in there, your worry is that your bio-load may be too high for the amount bacteria you have and it needs time to reproduce and expand its populaation. You simply added too many fish to a tank with questionable bacteria present. very risky, but what is done is done as Richer said. test ammonia, nitrites, nitrates, PH, and hardness at least once a day, and control any fluctuations with water changes. Keep a chart or list of test times and results so you have an accurate pattern. As was said, test before and after the water change and make sure you did a big enough change to keep toxin levels low. I have never kept stingrays myself, but I do know that they are said to be EXTREMELY sensative to water quality. They are absolutely awesome animals.
Good luck, I sincerely hope this works for you, and you never put yourself through it again.