I used to make subwoofer boxes with it. For water volume vs thickness formula I dont know.
I do know when it comes to basic drilling and cutting ..Heat is an issue...pressure and fracturing can happen.
Since we dont need to worry about the looks of the hole..cause your gona have a piple sticking through it... do try to undercut the hole and leave some meat behind to drumm sand with a finer grit to remove rough scratches (more of a cosmetic thing) take your time durring the sand process and periodically put the pcv or what ever tube you use through the hole till it fits...you want it very snug is my guess for fish tanks...from a speaker box prospective we did cause of sound and air pressure.
Small tricks with drilling and sanding...Get a bucket and sponge...with your off hand hold the sponge against the head of the tool be it the Bit, Sanding pad, polishing pad etc...this will cause a trickle of water to flow onto the friction area to keep it cool as you cut,drill,grind, sand, polish ..what ever. Ive learned this the hard way on my first speaker box.... talk about outer edge meltdown...i drilled so fast I melted OVER the diamter I needed...thank god it was fixable mistake on that end.
I am mainling giving tool use advise on plastics vs actual tank building here.... when cutting plastics...also using C-Clamps and brace the area you may bandsaw or jigsaw to prevent over vibration..(again use sponge and water to keep things cool.
Most bladed circular cutting things for drill bits or grinding tools for angle grinders can come in Fine tooth...try to get a fine tooth type blade like a hacksaw vs tree cutting edge that looks like sharks teeth also....
I stress this..undercut a little so you have room to sand and polish your edge work...
Polishing plastics was fairly simple....you work up the grit tree till you remove large scratches and small...and to rid yourself of the smokey look you need a buffing wheel and buffing compound... thats about it...
Your local Lowes or homedepot may have someone around the acrylic cutting section to help advise on this....(my lowes in tacoma has that section)
Just general advise here...Ive made many speaker boxes...more than I wish to recall...
PS..wear safety goggles and a mask to!
(EDIT)
Just read the link...dude has better tools than me...(cheater!) but nevermind...he covers everything well.