I definitely hear you, mbus are pretty picky on their parameters, I do, however, believe that they can be turned marine, but can never live in freshwater because they were born in brackish conditions. I have my 10g refugium starting (3 weeks into it, still having some nitrite and nitrate fluctuation). You could always try putting a piece of dried (dead) live rock to buffer the tank slightly- if he stresses remove the rock the rock and do a 25% pwc. The one great thing about 10g is how easy change outs are, the bad thing is they have to be changed all the time. I was doing 25% pwc a day plus 60% once a week when I was changing my puffer over in a 10g.. Plants, lol, you have a beautiful driftwood piece that could anchor down an anubis nana, hearty and relatively small- wait nvm, lol, I remember why I didn't have plants with my oscars- if you want to try in that tank stick with potted at first and watch and see if your cichlids play with it or move it around tearing at the leaves, they have a tendency of redecorating. I have a 10g planted tank with a betta and a pair of fiddler crabs. You could always look up mangrove plants, they're brackish friendly and usually pretty lush.
Gorgeous white elephant ear, that's a pretty rare fish you have there too... Lucky!!!
