The various reasons have been listed in posts prior to this one. I think you may be hyper-concerned about the potential aggression issues of mbuna. Secondly, if given the choice, mbuna males are going to desire to build their own beds in the substrate as far under rocks as they can get. Just an FYI.
Managing their aggression has more to do with selecting the right combination of fish AND the right male to female ratios for those species. For most of the species, 1M/4F is what is typically recommended. For more aggressive species (Melanochromis, Tropeops, Psuedo sp/elongatus types, etc) require a higher ratio of females to single male. Demasoni as an example are the most extreme with 1M to 12+F.
The other factor is tank length and EIGHT FEET goes a LONG way to increasing the margin of error relative to aggression.
As to your questions:
Thank you very much.. I am proud of it.Nice. Your tank is beautiful and very natural looking. You have a TON of fish in your tank.
Do any of your fish fight?
They quarrel, yes but nothing severe.
I figured having more hiding places would be a good thing so I wanted a backdrop that would provide me with enough hiding places for the fish and would allow me to have minimum things to remove when cleaning the gravel and for them to dig under. I also wanted to set up the tank so that it would encourage breeding. I hope that after its all in, painted it won't look extremely ugly. LOL
The PVC pipes will not promote breeding of mbuna as much as rocks/gravel & balanced community of fish. Their native environment is a rocky shore line. "Mbuna" means "rock dwelling" I never remove my rocks to vacuum the gravel.. The rocks are sitting on the glass with the sand surrounding them. I basically vacuum what I can get to. Do I get it all, all the time? No. But you don't have to every time either.
What kind of paint should I use? A black spray paint? Would it be toxic?
Yes, black spray paint. I prefer flat & you are painting the BACK of the tank, not inside. Not toxic.
How is your tank maintenance?
Keeping a tank pristine is a labor of love. It is a weekly routine, along with the water changes. you can make the process easier somewhat but IME it is just something that has to be done regardless of the fish you keep.
I was thinking of doing a sump. Recommendations?
Are you committed to drilling the tank? I wouldn't do a sump unless you are. I can't recommend hang over the back drain boxes. I have had them & can't stand them. Personally, I wouldn't have a tank that large WITHOUT a sump but that is just me.
There are pages in the 180 build thread I attached that goes in detail of drilling the tank & building the sump, the plumbing, electrical - everything.
That is a SUPER tank you have & I would really like to assist you through this forum. That thing done properly could be really, really cool.