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What's the water level supposed to be? I keep it 1.5-2 times the length of Prometheus' carapace. Is that right or did I just dream up those measurements.

You have the minimum amount, and it's not a recommendation. Water should be as much as your tank can safely hold and your RES can't climb out. They are superb climbers, so use a screen if necessary.

How big is Prometheus and your tank?

It all makes sense! I was confused because I thought that was the actual level the water was supposed to be at so I bought a tank (rubbermaid tub) on it's demensions rather than it's gallon capacity. the tank is 25 gallons and Prometheus is about 4 inches long. I bought the tank (rubbermaid tub) because i read that the tank needed to be 2 times in carapace in width, 4 times his carapace in length, and the water 1.5-2 times his carapace. In those terms the tank (rubbermaid tub) is just right but now that this has dawned on me...I suppose I'll buy a new tank (rubbermaid tub).

UPDATE: oooooook, the water is now at the level in which he must swim about 2 inches to get air although it looks like he has a little struggle to do so. He scales a wall and struggles to get up there but he nevertheless gets up there. I'm worried he might drown. How do you know if a RES drowns?

Low water levels (and other obstructions in the tank) can lead to drowning and I would suspect that a drowned turtle would be unconscious. For the record, never turn a drowned turtle upside down (it may cause the remaining oxygen to escape). I've also read that people would open their turt's mouth to drain out water and some even suggested to push the arms inside and out repeatedly as a form of CPR.

I've used a 40 gallon plastic tub before. Just remember that you will need to brace it somehow because it may bow out.

If you eventually make the water deep (support those sides, though), and you think he's having trouble initially, have some objects like plastic plants for him to cling to/rest on (shouldn't really be necessary for a 4-inch turtle, though).