If you want African cichlids (looks like specifically you're looking at Malawi mbuna?) you want filtration that equals 8-10x flow rate. So if you're looking at a 300L tank, look to get 2 or more canister filters that combined have a flow rate of 2400L-3000L per hour. So if you like Eheim Classics, get the 2217 model (600L/hr flow rate) and run 4 of them. I'm not sure on the flow rates of the Eheim Professional series so if you like those you'd have to look them up for yourself. If you like Fluval, get the FX6 (2130L/hr flow rate) and go with 2. It'll be a little overkill but hey, too much filtration is rarely a bad thing and mbuna like playing in the current.