I did a project very similar to this maybe 6 years ago... Those posts are on this site somewhere.
I'm surprised that simply filling and draining the tank will equalize the PH. If concrete is going to change the PH (because of the rock it's made from) then I wouldn't expect this to go away.
What I did was cover my concrete with a 2 part clear epoxy to seal it against the water. It was 95% successful.
After a year or so I noticed that the concrete was starting to leach into the water and create what looked like a calcium buidlup on parts of the wall that may not have had a thick enough epoxy coating. That was only one small spot and wasn't a big deal.
The big deal came from the actual layer of concrete that began to separate from the foam board after about 3 - 4 years. My thought was that the foam board is so smooth that the concrete eventually just pealed away from it. Now with more exposed surface area the resulting mineral buildup exacerbated this pealing. Any flat surfaces should probably be roughed up a little to create a better grabbing surface for the concrete.
Something to think about: the rock wall takes up space. The deeper your tank the better. You will loose about 2-6 inches of open space for the wall... which means the functional size of the tank is now smaller.
Were I to do this project again I would try to find an inert concrete or other water safe material to use. Also, you can buy tubes of aquarium safe silicon. Use that and not the standard tubes as the video showed.
One more thing:
Plan where your equipment is going to go. You may need to build your rocks around certain features. Where is your filter intake going to hang? Allow enough room around it for adequate draw. Where is your heater going to hang? You can not suction cup it to the rock which will force the heater to the side walls, or require you to get an undergravel heater. If you plan on running air tubing maybe you can carve a channel from the back of the foam and insert the tubing before the wall is in place.
Dry fit, dry fit, dry fit!
Good luck with your projects and I hope the above may help someone out.
Jonathan
