I would say that you do not have enough filtration. 400L per hour works out to about 100 gal per hour. On a 30 gallon tank, your only turning the water over about 3 times per hour.That's RATED, which is never very accurate, so your actually moving much less water. I would aim for at least double what you have.
30 gallon tank is pretty small for most mbuna. What type do you have now? What type would you like to keep? What is the dimensions of the tank?
30 gallon tank is pretty small for most mbuna. What type do you have now? What type would you like to keep? What is the dimensions of the tank?