What I am hoping:

The dry ice will sublimate inside the tank, building pressure. At some
point the pressure will be high enough inside the tank that this
process stops (untill you leak a little CO2 into the aquarium.

At what pressure does the CO2 reach this.......equilibrium? I have no
idea...... What's worse I don't know how to find out other than
experiment. So, with this in mind (don't laugh) I found some 6" OD,
1/4" wall seamless steel tube to which I am going to tig weld 1/4"
thick plate "plugs" to cap the tank. I Already have the round stock and
I have turned the caps on a lathe.......O.K. now you can laugh.

The tank weighs about 15lbs! and I figger it'l be good to about
Mmmmmmmm........275,000psi. In all seriousness, I'm not worried about
the weld till at least 1500psi. Irregardless, the first time I am going
to pressurize this "bomb" in the front yard.

What's really gonna suck, is if the CO2 sublimates to 15psi....at which
point it reaches this "equilibrium" and I now have a very interesting
looking boat anchor. I guess lucky enough for me I pulled all this
stuff out of the dumpster at work. So, so far it hasn't cost any money.

Hey, this might even draw a crowd! Maybe, I could charge
admission!!.............Maybe, not.