I am doing around 40% every 2 weeks. I figured it is less stress on the fish this way than every week. Being a corner unit I am pretty much forced to work from the middle and it doesn't give the fish anywhere to go. On a "straight" tank the fish can move to the one side and not be to interupted while you drain and fill from the other end. I have read so much over the past few weeks and so often there is always something to contradict the other. I have read that overcrowding is better especially with Mbuna to curb the aggression and heard figures of almost doubling the normal tank capacity with them. As mentioned by jweb1369 I think it comes down to filtration (hence the origin of the discussion) regular water changes and keeping the cannister in good operation. I will also test water weekly. The fish are still small SO i will probably bump up the filtration when they get bigger and improve the "turnover" period to about 7 or 8 times an hour. The other prob with corner units is there is not a lot of space to hang filtration ect together with inlets outlets heaters cables ect ect coz you are pretty limited to the bank end of the corner unit especially with the canopy I have that fits tight over the top, therefore almost having to resort to one filter and just go BIG with it. Thanx again for all the feedback and info it has helped in giving me a far better idea of what is going on and the specific needs Cheers Rodrick