The link showing the epoxy is applied over the pink foam, not over the crete. Call any manufacturer of epoxys and they will tell you it will not stand the test of time on top of cement, especially submerged. Unfortunatley, cement by it's very nature resists 99.9% of all topcoats, and that's ABOVE the waterline. West systems epoxy is only a polyester resin, which will not work long term. You would have to use a real epoxy resin, like this one:
http://www.tapplastics.com/shop/category.php?bid=2&
SURE you can match the chemestry at water change time. But due to the fact that the cement is being painted on and not troweled on like a real mason would do, the mix must be thin instead of very thick. I don't care how many "painted on" coats you apply, the cement mix will always be weak from the get-go. All desireable strength properties of cement are compromised severely when such a large ratio of water is used in the mix. It will now
most definately be attacked by the low Ph, soft, acidic water and will spall, check, crack, flake, chalk, erode, etc. VERY rapidly... unless you apply a "low-slump" mix with a trowel over the first painted on coat, it will lose its integrity fast. This means that it will
BE DISSOLVING IN YOUR TANK, right before your eyes, utterly destroying your attempts at water chemestry balancing.
I am not guessing on this, it happened to me. I added a MILD acid used for cleaning excess grout off of ceramic tiles to the water to see what would happen as a test. I brought the water down to about 7.0 ph (started with a ph of about 8.0 in the water the cement was curing in) and within a week I had all this sand on the bottom of my tank that was not present before (it was bare glass before the sand was there). It was all the fine aggregate from the cement mix! The top two out of four painted on cement layers had dissolved away, and only the silica sand from the mix remained, and you could punch right through the cement down to the foam with your fingertip like it was tissue paper in most places! I drained the tank and applied a thicker layer with a trowel wherever I could and am curing it again as we speak (wondering why I even tried this, as
that layer is gonna take a long time to cure.. but it looks great). So I am not speculating here, I am telling you the facts of what happens to thin watery cement layers when even neutral ph water hits it. The bare cement will only last as long as the water it is submerged in maintains a high enough ph.
The sika top seal 107 in the above link looks very promising, but is it even sold in the US?
Triazole says it all s
o eloquently, the reasons to rethink the discuss... I also say scratch the discus idea and go with a hard water, high ph loving species for your custom background tank and relax and enjoy the friuts of your labor...
then get another tank for the discus!
to move about. (believe me, they need all of that miserly 12" the 55g's provide) 
