We may be missing the point a little bit here

Obviously most fish keepers would like a larger tank than they have but for many reasons, such as space or budget, may not be able to.
That being said that doesn't mean they can still keep what ever fish they desire in the tank they do have
Many of us try to push our luck and kid ourselves that fish in an undersized tank 'look' happy therefore are. (I have yet to see a fish smile!

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It's safe to say that a 125lt is too small for Malawi and is borderline for most Tanganyikan Cichlids. A couple of examples of stock have been suggested for Tanganyikan and there are more I'm sure.
When you see Cichlids in the LFS in the 2ft holding tanks it's tempting to think that coming from there to your 125lt would be an upgrade for them and they would be fine. Not so

While in those tanks the filtration is is massive for a start but more importantly the fact that there are so many fish in there with no rock-scape it's imposable for them to claim a territory.
One of the key interest in Cichlids is their interaction with the rock-scape followed by their 'attitude'. All this is based on claiming a territory for breeding. Malawi Cichlid keepers tend to over stock slightly to lessen any one fish claiming too much space which is meant calm things. This you mustn't do with Tanganyikan Cichlids as it causes stress.
Now, back to your 125 lt. You now may be forgiven, after reading this, for thinking ok I'll just go with Malawi Cichlids and what ever I put in there will mean it's overstocked therefore all will be ok. Again, not so
Cichlids require over filtration, especially if overstocked obviously, so an internal for that tank would be inadequate. Finally the final 'spanner in the works' is they will get very big, very quickly.
So, to sum up I'm afraid it is a larger tank or forget Malawi's
Great suggestions so far have been offered so good luck
