my recommendations to those who are fairly new to rays:
1) watch your pH- the bio loading for rays is high and once the buffering capacity of the water is exceeded the pH can crash from 7 to 5.5 in hours. that kills.
2) watch your nitrates- again, the bioloading is high. plants and denitrators aside, i personally advocate 50% WC twice a week.
3) heaters- i personally save heaters only for weak pups, not for stronger pups and adults. if you must use them, use a heater guard.
4) comming rays with rays- there is no fixed formula for mixing rays of this size and that size, or this species with that species. the golden rule is- You need to observe them. One thing is for sure, stress from harrassment kills a ray much faster than starvation does.
5) comming rays with other fish- general rule of thumb- no plecos, no feifengs, no pig nose turtle. Of course there are exceptionally ray friendly plecos, so remember the golden rule- You need to observe them. Aros that have grown up in comm tanks are fine. A 4- yr aro who has been alone in a tank all its life would be less friendly. Try not to comm rays with fish that can fit in its mouth.
rays aren't the most difficult fish to keep, but they arguably are the most beautiful.
enjoy!