I've been hesitant to respond to your post. Because basically, I wasn't sure what you were asking. I mean sometimes people just want lights that make their fish look good (*ahem* The African Cichlid crowd?).
But, I think your lighting interest in this case is a bit more well, interesting. That's because there are multiple parts you can unpack on this question.
So, lets see where this goes....
- Purposely Growing Algae. Yes, I've done that. And yes, those plant growing lights you have are definitely gonna help promote algae growth in the tank. The idea being of course, to grow some free 'nibbles' on the rocks for your Mbuna. Be Advised: There is a cost to that idea though. The Mbuna will certainly graze on every possible fleck of green color they can imagine on the rock pile. On your 3D background (if you have one). On the aquarium glass? Oh, not so much.... If you want to grow algae, beware what you are asking for. You WILL get it! Weekly (or more) glass scrubbing chores to remove algae buildup on the glass, could be an additional part of the regular maintenance routine.
- Will my algae-growing lights be too bright for the Mbuna? In a word - NO. I mean sure, for up to a half hour or so when they flick on, your fish may sulk in the tank for a bit. Soon enough though, they'll be out chasing, nipping, lumping, bumping and doing well, Mbuna things. You know that the best way to grow plants (and Algae) is to place your high-performance lighting on a timer system, right? So, have your 'high beams' shut off to less eye-scorching levels when YOU want to see all of that non-stop Mbuna activity and excitement in the tank....
- Which Rocks Grow Algae Best? Oddly enough, that is one question that isn't given much consideration. And should be, if you seriously want to encourage algae growth on the rocks stacked in the 'pile'! So what doesn't work very good? Lava Rock! And why is that? I'm not exactly sure. But, I do know that Lava Rock will leach out alkaline solids in a freshwater tank. They are also high in iron. These are things it seems that algae doesn't like much, or just needs a small amount of. The smooth landscape rocks that 'DJRansome' likes best, grow excellent crops of algae. Oddly enough, pieces of 'Texas Holey Rock' can get green pretty fast without Mbuna to detail clean them - maybe it's the very white color? Mountain Lace Rocks do okay as algae growers. Maybe the rough surface (Like Lava) makes it harder for algae to get started on them?
In ending: I like your old plant-grow lights. In the algae growing application you intend, they should work pretty good. I would recommend getting some softer, dimmer lighting to GO WITH your high-powered grow lights. Set up and use timers for all your lights. Set 'em to your schedule, so your tank is lit up to look its best when you're home to enjoy it! :thumb: