If you're thinking in terms of "minimum aquarium size" when it comes to stingrays, you're doomed to failure. Let's be really clear about this. Stingrays are incredibly expensive fish to house, both in terms of aquarium size and in the necessary investment in RO filtration to produce the nitrate-free water you need for water changes. If you haven't already bought one of the several good stingray books, go and buy one, now. Richard Ross' book from Barrons is excellent, but the Gonella and Axelrod book from Interpet is pretty good too. Both are cheap; in fact I got a copy of the latter for one whole pound at the Maidenhead Aquatics store in Oxford.
Anyway, for what it's worth, the guideline is this. Choose a tank at least twice as wide as the disc width, and four times as long as the disc width. For a motoro ray, that's going to be about 90 cm from front to back and 180 cm from left to right. Smaller fish might be kept in smaller tanks, but you'd be unwise to start off with anything less than 60 cm from front to back and 150 cm from left to right. Buying a smaller tank isn't sensible in terms of cost because you will also need massive filtration and, I'd strongly argue, a sump as well, so that the tank has lots of water to dilute nitrate and buffer against pH changes. So you may as well buy the big tank right from the start.
Cheers, Neale
1 of my lfs has 2 rays in a display tank and they have had pups, as far as im aware they are motoro rays. i was talking to the shop owner and he said he will sell me 1 for £75 ish when they are a bit older
what would be the minimum tank size for a pup and eventually an adult?