I agree with Jeff, "don't" clean both filters at once, do one, wait a while, then the other, and step up water changes gradually. If you get at all distracted, while cleaning both filters, you could wipe out all the beneficial bacteria, and end up with an ammonia spike, and a dead tank.
I also agree your tank is overstocked, and that, that overstocking is contributing to the nitrate problem.
Whenever you acquire young cichlids, of course growth is obvious, but I doubt many new aquarists realize, water changes must be increased with and coincide with that growth.
The same changes that were adequate for 2" cichlids, are not even close to being adequate for 5" or larger cichlids, especially with the over population you have.
I also agree your tank is overstocked, and that, that overstocking is contributing to the nitrate problem.
Whenever you acquire young cichlids, of course growth is obvious, but I doubt many new aquarists realize, water changes must be increased with and coincide with that growth.
The same changes that were adequate for 2" cichlids, are not even close to being adequate for 5" or larger cichlids, especially with the over population you have.