You need to secure them to the wall somehow, as you should do with all large furniture items.
If you have a glass tank, your best bet is to secure the stand it sits on to the wall with earthquake straps, and find a way to potentially lock the tank to the stand. Or, you can get high strength steel wire to wrap around the tank, and secure that to the wall if you are really concerned about it. you cn get surprisingly thin braided steel line and depending on how you do it, you might not even hardly see it wrapped around your tank (if it's tucked up to the top where the plastic casing is.)
A glass tank will be no more dangerous than a wooden tank unless the tank falls, or if something falls on it. The advantage a wooden tank has, is usually the lighting is securely mounted to it. So you'd want to figure out a way to also secure your lighting.
This is all good timing to focus on earthquake proofing your home, knowing where your water and gas shut offs are, and preparing for what you need to do, if one strikes hard where you live.
We moved into a new home last year, and it was a nice reminder for us that we have done nothing to prepare for what to do if a quake hits us hard. So it is good to create a plan of action.
-Brandon