This is a myth. Central American and Rift Lake African Cichlids require the same type of water. South American fish tend to need soft water and that is the difference you have to watch out for. Central American and African cichlids do great in tanks together. I make this mix often. Your filter is not really adequate for the big fish you keep. I would suggest saving up for a canister or adding at least one more filter like you have now. Personally, I have a low opinion of Whisper filters. I prefer Marineland, specifically the bio-wheel filters, when I use a power filter. FOr canisters, I like the Hagen ones.