How many yellow morph did you add? If all of them were dead around the same time, it could mean they were not able to adapt to your water. The water hardness, PH, temperature etc. might be to blame.
Did you acclimate them slowly to give them time to adapt when you introduce them to the fish tank? This is extremely important every time you introduce new fish to the aquarium. Large difference in any of the 3 (water hardness, PH, temperature) can quickly shock and even kill them if you did not give them time to adapt.
When we add new fish to an aquarium, we need to first let the plastic bag sit in the fish tank water to allow the water temperature to be the same first. Then we add some of the water from the fish tank to the plastic bag. Wait 15 minutes or so, then add some more. If the plastic bag is too full, then dump some of the water before add a little more from the fish tank. (Do not dump the water from the plastic bag into the fish tank). Just repeat the process every 15 minutes. Until the water in the plastic bag is almost all from the fish tank. This is called acclimate. It can take anywhere from 1 hour to 3 hours or more. Skip this step might kill the new fish.
If your cichlid and catfish were doing fine, then the water itself was fine. However, since all of the remaining fish are dead in there. I would do large partial water change. Maybe somewhere around 70% of the water, twice in a row, or just one 90% change.
No need to take everything down, as it will crash the aquarium nitrogen cycle.
On a side note, do not be discouraged. You might also want to add new fish within a day or two. As there is no more source of ammonia, the aquarium nitrogen cycle will slowly crash on its own even if you do nothing. The new fish will keep the aquarium nitrogen cycle going.
