Is sealing concrete necessary? No. Would it be the more prudent approach? Yes.
Half of my home has stained concrete floors. I made sure I tried various techniques to stain and finish concrete before I attempted my own floors. I design and build homes for a living so I have a little firsthand experience with mud. If you are going to seal the concrete make sure that it has thoroughly cured. Tape a 2"x2" piece of plastic to the concrete surface and leave it overnight. If there is any moisture trapped by the plastic the following day then it's not ready to seal. Continual 'wetting' of curing concrete after the first 48 hrs has minimal benefits.
If you are not going to seal it, soak it in a garbage can of water for about 3 to 4 weeks. Aerating and weekly 100% water changes help while this soak is going on, as does a bit of muriatic acid. After that PH should not be affected in a high PH tank for african cichlids, but may still swing up in a neutral or acidic setup.
Cunados: concrete needs moisture to cure to full strength. To cure properly it needs to be kept wet for 28 days. However, this only makes a difference in structural applications like walls and bridges. For floors and DIY backgrounds you are right.