As Ch4rlie was hinting at, we need to see the numbers for water tests. "Perfect" can have very different meanings. It would also help to know the species of the 4 small cichlids (those dead and the survivors), and did you have any fish in the existing tank that you moved over? In other words, which fish had fungus, which were aggressive, etc. While waiting for this data, I can offer a couple suggestions in general.
These decorations, were they from the place where you acquired this used tank and fish, or yours from the existing tank? They were probably covered in various bacteria, mostly good, and this can help establish new setups. If from another source, maybe a different approach.
There is nothing beneficial in old tank water, so using all fresh is generally preferable. What is in the old water is detrimental, though not dangerously so if it is your own tank. But the good nitrifying bacteria will not be in the water, it colonizes surfaces (like the filter media, substrate, wood, rock, decor, plants). The only time using the old water may be beneficial is if the parameters differ significantly, to avoid shock to the fish. And on the cloudiness, this is probably a bacterial bloom, very common in new tanks or those in which there has been significant "cleaning" of the filter, substrate, and sometimes water changes alone. Provided ammonia remains at zero, I wouldn't worry over the cloudiness, it should clear sooner or later.
On the Melafix, I am not a fan of these generic "cure all" sort of remedies, as they are often not effective and even more often may be detrimental. If a disease is present, the safest and most effective treatment is one relevant to the disease. If would help to know just what this "fungus" was; for example, you mention aggression, so was the "fungus" on the fins, suggesting it could be due to fin nipping?
Last point here, on the water changes. On a general routine basis, change more volume, closer to 50-60%, once weekly. This is actually more beneficial that two smaller changes, as the larger volume removes more "pollutants" at one go. This applies to all fish, but cichlids are especially aided with larger changes, as they have a real sensitivity to water conditions.
Byron.