I would not recommend the glass method at all. That is a very good way to risk losing a camera. If you are willing to take the risk go for it then, I know a few of us have cameras that are not easy to replace.
Here are my tips:
1. Room MUST be dark, you need to minimize non-fish tank light sources as best as you can. Otherwise you will have reflections on the glass. Reflection + auto focus = no good.
2. Stay still. Don't chase the fish. #1 reason fish come out blurry is because they are moving too fast. Track them with the camera steadily and take a pic the second the fish slows down.
3. I don't suggest flash, it distorts colors anyway. But if you need to use flash I honestly suggest the opposite of what was suggested before. If you use flash stand way back from your tank and zoom in. If your camera doesn't have zoom then your on your own with flash.
4. Fish tanks are poorly lit which makes photography hard. Especially since fish are uncooperative fast moving targets. Best I can say here is try to use the "action" or "macro" setting. For those that are familiar with manual controls, use a low ISO and a shutter speed of 1/20sec. or faster if you can. Inga I'm confuse why you suggest high ISO... sure gives you lower light abilities, but it shows a lot in pictures =/. I almost always shoot with ISO at 100, Aperture wide open, and then slow down the shutter as much as I can before things go underexposed. If I still need it faster then I will let off the ISO tiny bit...
5. If there is some way to increase the overhead lighting on the tank with a desk lamp or something temporarily this should help a lot.
6. IMO use auto or manual focus, what ever one works best for you. Some cameras have very bad auto focus and work best manually focused. I barely know how to use manual focus on my camera. The auto focus works of me or I can easily trick it.
In the end though, the camera and user can make all the difference. It does take awhile to learn the basics of photography, and I don't even think I have mastered them yet. Anyone can do it though and once you know what you are doing quite a few cheap point and shoots can be pushed to their limit