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Hi,
What's the minimum required water for a big adult?
If we want to use plastic instead of glass tank- do you have any recommendations on where to purchase it?
TIA

10 gallons per inch of shell length. On average/minimum, a 55 gallon for a male RES, and a 90 gallon for a female. You want what's called an acrylic aquarium. Those you should check with local aquarium hobby shops for the nearest place to get them. Just be aware that they will cost a lot more than an equivalent glass tank.