First, your diagram seems to show the fish looking smaller. You could call that magnification by a fraction, but that doesn't sound like what you meant.
To make it look bigger, the glass (acrylic) needs to curve the other way - convex on the air side.
Both the curvature and the distance of the fish will affect the magnification.
Quick calculation, I'd say max magnification when the fish is 4x further away than the radius of the glass. That puts it at the focus of the lens made by the curved water surface. If the fish is nearer the side, it will be less enlarged. If further away it will be out of focus until it is far enough away to make an image between the observer and the tank. If you stand well back, this could be even more enlarged, but upside down.
My guess is you'd be best served by having the focus around the farthest point you'd want to look at the fish. If that's the far side of the tank, then the radius of the side would be at least a quarter of the width (sideways depth?) of the tank. But the greater the curvature, the more distortion you get and I'd guess you wouldn't want more than 20 degrees of arc over the side.
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Having a cylindrical side will only magnify in one direction, distorting the shape of the fish.