I agree with having the 20 long instead of the high.
From my experience, the more floorspace you have, the more caves and rockshelves you can put in. They need that type of stuff. The more the better.
Keep in mind that the labs can get up to about 4-5" and may eventually need a new, larger home. The shell dwelling labs are great for a smaller tank though. I had a spare 10 gal
that I wanted to put something in (still using as a QT
tank for now though; saved some lives with it, decided to keep it). Here's the thread with some really good links contained within...
Another option would be just using a bigger tank... (african rift lake cichlids are great fish when you have ample space for them...)
I use a crushed coral substrate to raise my Ph and hardness. Out of the tap my Ph is 7.0, with the substrate it is around 7.8 I wouldn't advise using chemicals to change the Ph. Even though they will indeed change it, it is another thing that you will have to...
1. invest in (gets pricey real quick like)
2. potentially mess up (losing count of how many drops you just put in)
3. have things out of your control (what if you got the bad batch?)
4. using a substrate that keeps your Ph high is a self sustaining option
Basically the less chems in the water, the better (IMO
Argonite also raises Ph, but is more like sand (I like my coral and so do my fish)
Anything that is some form of calcium carbonate will work, texas holey rock (not a substrate) is great for africans; lots of caves/holes and it's a big ol' hunk of limestone (raising the Ph).
Keeping the water conditions right is actually the easy part (as long as you keep up on regular maintanence). From my experience, the trick is dealing with thier rambunctious behavior