Rocks for a Malawi tank

Rocks are a bit subjective, and can originate from a variety of sources. The bigger question is, APPLICATION.
That is, what sort of African Lake Malawi Cichlids do you intend to stock this aquarium with?
For African Mbuna, that will entail a higher, more substantial rock pile in the tank to replicate their natural living conditions in the lake. Plus, the larger amount of rock helps to mitigate aggression problems by breaking up sight lines inside the water column and provides chased/harried Cichlids a means of escape from the more aggressive, dominant ones in the aquarium.

150 Gallon African Mbuna Tank, ca. 2004. Almost 300 pounds worth of very large pieces of Mountain Lace Rock were furnished in this aquarium.
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36 Gallon African Mbuna Tank, ca. 2005 (stocked species-specific, w/Chindongo saulosi only). Landscaping rocks were furnished in this one from a local garden center. The diagonal 'bogwood' piece in the middle is actually a large piece of cast resin.
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For an African Haps Tank, a lower, less dominant rock pile in the aquarium is preferred with more open sandy areas of the tank to swim over. That effect is shown pretty well by the rather nice Lake Malawi video posted up by @dstuer of the sandy bottom areas in the lake, for your earlier thread about substrate choices for this new aquarium.

Okay then....

djr955 said:

How high do you have them stacked? I saw some nice naturally rounded stones at a LFS but thought they might be a nightmare to stack securely.

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It helps in building the stack, if you can find stones with a rounded profile that have a more coarse, rougher texture on the surface.

Auballagh said:

Don't get too hung up on getting only the 'best' rocks, or other things. The right rocks should be reasonably cheap. Fairly smooth in edges and surface texture, but not so smooth that they will tend to slide on or over each other when in a pile.

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For my own African Mbuna tanks, the rock pile was built up almost the entire way up to the surface. This image from a Central American (Riverine) biotope aquarium I recently built, may help to explain that concept,

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And no... the large bog wood pieces in that setting, DO NOT apply to your African Rift Lake build! But, you can see the visible effect of building structure up to the water's surface (and even beyond) in the aquarium. The visible rocks used in building 'the stack' are just rounded, landscape stones purchased out cheap at a local plant nursery supply store.

Hmmmm....

djr955 said:

If I stack a lot of rock in the tank, is that reducing the number of fish I can keep as its displacing a lot of water volume?

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Well, not necessarily. Meaning? Those two things aren't exactly interchangeable.

• With a lot of rocks, the length and width (esp. the length) of the aquarium are still in play. Factors that really do decide the stocking numbers for the tank. And, having more rocks/structure furnishing an aquarium, can actually ENABLE more stocking numbers as aggressive (bully) fish can't see their victims as easily with those sight lines broken up, and harried/bullied fish can more easily escape with things to hide and swim into out of sight.
• But yes, a lot of rocks WILL displace water out of the aquarium. This can reduce volume, making water quality harder to maintain in heavily stocked tanks.

So, done right the rock thing is kind of a balancing act. Possibly the 'best' rocks I used for building out my early African Mbuna tanks was that Mountain Lace Rock. That stuff is thin in profile, has a coarse/rough surface texture and comes in some pretty big sizes. That enabled me to safely stand those big rocks up on end, building these high 'walls' of rock in the tank.
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EDIT: Oh, and if your water is pretty hard (strongly buffered), it is doubtful you will get much PH reducing effects out of any bog wood placed in it, beyond simply tinting the water. The water acidifying effect of bog wood in aquariums has been greatly exaggerated over the years, and has become almost an aquarium myth at this point. So, if you do have reasonably hard water and would like to furnish your African Mbuna tank with an interesting piece of bog wood or two? Do it! The stuff will probably do almost NOTHING to affect your water chemistry.