another thing i heard is to pore vinegar on it and if it fizzels then it shouldnt go in the tank

the fizzing thing with vinegar will tell you weather or not it will affect your water chemistry as alkaline rocks will fizz when acid is added it will push your ph up a good thing in a Malawi tank a bad thing in a soft water tank

oo and have a read of this good article lots of ideas and mentions the clay pots and pipes

http://www.cichlid-forum.com/articles/lake_malawi_setup.php

Yes there's some fizzing going on with the brittle rocks, so they must be alkaline. That article is great gwa, thanks for that! I did wonder about my tank not having sand, and that article says I should have sand but my tank has a gravel (think its coral substrate).

ye i use coral gravel buffers the ph of the tank nicely mine is very fine grade tho almost sand but not quite

but ye terracotta not clay pots is the way to solve your problem with aggression short term you could always cover them with rocks or slate slabs later

you can also use PVC piping cut into short lengths. I put 3 lengths together forming a pyramid shape using silicone sealant to secure to a piece of slate (to weigh it down) then using sealant again camouflage the whole thing with small pebbles or more slate/gravel etc.

Rob.........