Thanks for the comments!
No, it was actually quite dark. I used wide aperture (like around f/2.8, cause below this my 50mm lens is not very sharp and additionally the DoF gets drastically low), high iso, and still couldn't get a reasonable shutter speed in that light. These jellies move pretty quickly, so if I wanted the photo not to be blurred, I had to underexpose it slightly - it's the water that gets underexposed in this case, since the jelly is mainly white. Also, the water is solid blue, so easy to fix in Photoshop. (I recommend that aquarium, beautiful clean water, with this artificial intense blue color and backlights, as compared to other aquarium I visited, where water had normnal color and was full of food, little organic speckles, everything - that looked like snow or dust). So my postprocessing was to brighten up the photo, crop it from one side and add a bit of space from the other side. The original frame was centered, since that's easier to focus when you have quickly moving around jellies, and tourists, and you want to catch the moment when the jelly is separated from others and nobody's head or hand touching the glass and smearing greasy fingerprints on it is between your lens and the jelly.
PS. Terry, that worked. Thanks a million!