You are right, motoros do not tolerate any amount of salt. They also do not tolerate any ammonia or nitrites. They are one of the more difficult freshwater species to keep, but if their basic requirements are met, they aren't difficult. First you need a large tank that is at least 24 inches wide, preferably wider. Once full grown their discs can measure 36 inches across in the wild, but in captivity they typically stay under 24 inches. A 300 gallon tank or larger is preferred for a full grown motoro. They like softer, slightly acidic water (pH around 6.0-6.5). The pH isn't as big of an issue as long as it is kept consistent. Rays have been kept and successfully bred in hard, slightly alkaline water. They prefer temperatures of between 78 to 84 degrees F. I keep a pair in a 210 gallon aquarium with a pH of 6.5, 79 degrees, and feed mine night crawlers, silversides, chopped up whole, raw shrimp, and strips of squid. If you properly care for them, they are very entertaining and easy to keep.
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