20 gal vs. 30 gal in-wall tank ... need help quick...thanks.

I would go with the 30 gallon because bigger is better and more forgiving with regards to human error!
Do you have much experience with saltwater, and are you going to be the one servicing the tank and doing the upkeeping? You don't need to have alot of rock for the tank to be successful. For a 30 gallon tank IMO get say 15 pounds of live rock, but make sure you have a deep sand bed(4-5 inches) plus you'll save some money!
You really aren't going to be spending much more money based on tank size as you are only talking about 10 gallon difference, than if you were going from a 75 to a 125 tank.
For corals, what kinds do you plan on keeping? Mushrooms, polyps and leathers or going with stonies and more difficult types?
I don't know how indepth to go with this as I don't know how much experience you have with saltwater tanks!!!
Basically, you don't have to spend alot of money with the size tanks you are talking about(which ever one you go with). Just take you time to set it up right and have all the equipment before hand...maybe do alittle research on setting up a reef tank(everything involved ie. cycling, fish compatibility, water parameters, corals and inverts husbandry etc.)
Good luck with the tank set-up! I'm sure it will be a big hit with both children and children at heart!

 

I have kept a nice 20 gal reef in the past so I do have some experience with taking care of such a tank. However, it has been about 5 years since my last tank and now I am going to start enjoying the hobby again.

I think I am going with the 30 gallon tall tank. It will be 24.75 inch tall, 24 inch in length, but only 12.5 inch deep.

I don't know how high I should stack my live rocks since I am afraid that the kids in the waiting room may tap on the tank and cause the rock to fall down. I am thinking of putting a frame in front of the tank so that the kids would not have direct contact with the tank but the problem with having 2 layers of glass is that it will be hard or impossible to clean with a magnetic cleaner since the 2 magnets will not stick to each other through a thicker layer.

 

You can always use plastic wire ties or glue to stabilize the rocks to reduce the risk of them falling.
You can purchase a stron magnet to clean the glass. 220 gallon glass tanks have a thick glass roughly 3/4 to an inch in thickness and they make magnets for that size tank, however it is expensive.

 

my vote's for the 30 gallon tall.....it would look cool in a wall and you would have plenty of aquascaping possibilities (with the help of some epoxy)...