I have been keeping my 3 motoros now for 4 weeks. Brought them from some bad situations to looking healthy and growing rapidly. One of them was curling badly, but fine now. All of them are skitish, but are slowing becoming more and more used to everything. Three of them went through 1lb of blackworms in about two and a half weeks, now I am trying to get them over to shrimp. Here shrimp are cheap straight from the fishmarket. I have cut up worms and soaked them for an hour in with chopped shrimp in a small amount of water. They seem to like the worm blood smell better then shrimp and this kind of tricks them a little. Once they are fat and healthy starve them for a couple of days and they will usually switch over when they get hungry enough. This is what all my local friends do, and some of them have 12+ rays in their possession, all growing and healthy.
On the pH situation, don't worry about it. Unless you are trying to breed a sensitive species like discus or tetras don't bother. I have been keeping fish for 20 years now and generally you harm more with the pH fluctuations then just using tap. I am a Marine Biology teacher and we breed cichlids (africans, south americans, and central americans), danios, livebearers, bettas, gouramis, goldfish, and tetras. The only one we have to mess with the pH with is the tetras. I have friends who keep discus in their tanks without messing with the pH here. Our pH is 7.8-8.2, and our hardness is moderate. Everyone gets used to it pretty easily.
I do 60% water changes weekly, which helps the stingrays a lot. If you do the math then one 60% change takes out more pollutants then two 30% changes, and my rays seem happy every time a big water change happens, cruising around looking for food. Just make sure the temp is the same or a degree warmer and they should be fine.
Arowana will love the extra water changes too.
