I owned gerbils for a number of years and I never did things exactly by the book. My gerbils lived to or past expectancy.

1. Can a gerbil live in a fish-tank, with clear glass and no hood?
It's not ideal because of ventilation, but I did it. My gerbils never had respiratory problems, but it doesn't mean yours won't. You should get a top. You'd be amazed how high the little guys can jump sometimes.

2. How often do you need to clean their tanks/cages?
The tank will get pretty stinky. It's been a while since I owned gerbils, but I think I changed them once every 2-3 days. In a tank it will be easy to clean. You can also try removing the wet bedding and adding new bedding to the top between cleanings.

3. Can they live alone or do they need a pal, which is better?
I had some that lived alone, but 2 females will be about the same amount of work and then will have companionship. Two males will fight more, but I had 3 males living together with no problem, but they were brothers that were raised together. (I kept my pairs entire first litter, which was 2 girls and 3 boys. The boys lived together and the girls lived together.)

4. How long do they live for generally?
I think they say 2-3 years and mine lived about that. Usually mine went over expectancy. You can tell when they are getting older. They slow down a lot and get fat.

5. I heard of "wet-tail" and my friends gerbil died of it, what EXACTLY happens when this occurs?
I have no idea. My guys never had it.

Gerbils are a lot of fun.