which is better in a 20 gal?

Sorry i don't really know much about tangs. I do know that a 20 gallon is fairly small- but would be suitable for tang shelldwellers.

A small colony- maybe 4? Of shellies would work well in a 20. Mind you, a 20 'long' rather than 'tall' would be better as they live in the bottom region of the tank.

You seem to have done some research which is good, and someone who has experience with those species should be able to give you a better answer. I would stick to one species though, and only a few, because a cramped space will make fish more aggressive.

OK tangs aren't really my thing but i do kept shell dwellers.

I dont' think a 20g is going to be big enough for Neolamprologus Brichardi or Neolamprolo Leleupi( http://www.cichlid-forum.com/articles/n_leleupi.php ). At this link is says a tank no smaller then 55g is recommended. I beleive it is due to them being quite aggresive.They are technially small enough to fit in a 20g but They will fight.

Lamprologus Specious,N. multifasciatus, and L.ocellatus(i own these ones) will do ok in a 20g but only one species will work. A small group will work. I tried to keep 4(1m/3f) L.ocellatus in a 10g but after 2 paired up the left over females were very badly picked on and had to be moved. IN a 20g depending on your fish a trio will probably work,maybe 4(smae for Lamprologus Specious). N.multifasciatus is smaller and is a harrem(sp?) breeder so a bigger group of these will work. I would say maybe 6(1m/5f). When they start to breed the older fry take care of the younger fry so you dont' have to take them out to raise them,the parents wont' eat them.

I would suggest getting some shellies for this tank, you won't be dissapointed. Mine are pretty entertaining when they are moving their shells. They also are very feisty and sometimes its scary to put my hand in teh tnak as the female will attack it. They are also very easi to breed. Mine started spawning 1 week after getting them and have produced a new batch every 2 weeks.

Here are some good articles http://www.cichlid-forum.com/articles/shelldweller_corner.php

With good filtration, nothing should really settle on the bottom (at least it doesn't in my planted and tang tanks) so you don't really have to worry about vacuuming. The play said is fine, you'll just want to rinse it in warm water thoroughly so it doesn't cloud your water.

I would say the shells should be a little bigger than a golf ball. An adult fish should be able to fit in them.

I gravel vac my sand if it is showing lots of debris.I use my old gravel vac(the kind that you have to suck to start teh syphon) as it doesn' thave th power of my python. I jsut hold the vac about the sand(about 1") and swirl it around to make the debris start floating around, i then jsut suck it up. I have also gone into the sand and it doesn't really get sucked up the hose so it really isn't a problem.

As for shells. I jsut went to the LFs and asked if they had any old snail shells. I have shells ranging from 3/4"(the opening of the shell) to shells with 1 1/2" openings. That way the smaller females and fry get the smaller shells and the male can use the bigger ones.

Lots of good advice above. Shellies would probably be the best choice for this tank. You could have N. brichardi if you could find small enough ones, get 4 or 5, when a pair forms all but the pair will be hovering in the upper corners of the tank. Obviously, the 20 will only hold one pair. They're like the N. multifasciatus in that they don't eat their fry and their fry will then help raise the next batch. I know lots of folks keeping brichardi in a 20 (but long really is better for them) - you just have to cull the fry more often.

Personally, I'd go with multis. I had a 20 high that started with a group of 6, one of the six was exiled to the other half of the tank bottom (a male vs. male conflict), and eventually when there were fry from the colony of 5 the lonely sixth fish got a girlfriend...so I then had to colonies both with fry. There was constant border patrols, and lots of sand digging (I vacuum sand - I use fine argonite sand and just put my thumb over the syphon tube so it doens't pull the water through so fast). Multis are bulldozers. I'd ofter put the sand back flat just to watch them dig it up again - loads of fun and they are never boring. In fact, I wish I'd never sold off the colony...

Eric