Acrylic tanks can be a non-issue or a difficult-to-impossible surface through which to shoot. When preparing to photograph through an acrylic tank, your first step is to look across the front panel. If it's bowed out, which is usually the case with lower-grade/cheaper acrylic tanks, you will have a difficult time achieving undistorted/in-focus shots. If there is a slight bow and the coral is close to the glass, you have a chance of getting a decent shot. Any animals that are very far into the tank will be distorted/out of focus because the front panel acts as a magnifier. If I see any bowing in an acrylic tank, I don't even bother getting the camera out of the bag because it won't be worth the effort. You usually have a better chance with higher-quality and/or larger tanks, because the front panel won't bow out much or at all. With those tanks you can shoot as if shooting through glass.
I don't consider the clarity of acrylic to be a factor because it's very little light gain, compared with conventional glass. Usually the biggest problem with acrylic tanks is scratching, particularly micro-scratches. Often times they are overlooked until you start doing post-processing work. They can be noticeable as scratches, scratches with algae in them, or can cause a slight haze if there are a lot of micro scratches.
Difference between 1/2- and 3/4-in.-thick acrylic? Not in terms of whether you can get a sharp image. If it's good quality, unscratched acrylic, that isn't bowed, the shots will be fine. The only consideration is that, as the acrylic gets thicker, your lens face must be almost absolutely parallel to the acrylic face. Very much movement out of parallel will result in distortion and likely some rainbow effects in the image.
This image was shot through a top-quality acrylic tank.
Gary
