New Tank Zebra Danio Deaths
I just recently started this fishkeeping hobby, and began to stock my first tank a week ago. I have a 37 gallon tank (fully cycled) originally stocked with 6 short finned zebra danios, 3 long finned zebra danios, 3 albino cories, and 3 pepper cories. I was planning on adding a pair of Gouramis to "complete" the tank, but my danios seem to be struggling.
One of the short finned danios died the day after being introduced to the tank. This morning (~4 days later) I woke up and two more had passed after showing some concerning signs. They seemed to "drift" in one area at the bottom of the tank, mostly just hovering and swimming in slow circles. While I am far from experienced in knowing what to look for, their bodies appeared healthy and didn't show any obvious sign of disease. However, after dying, one of the danios had a large spot on it's back that I am assuming could reveal what exactly the fish was struggling with.
I do believe I have made a few mistakes in the introduction of these fish, and was wondering if this could have possibly contributed to their deaths.
1. The temperature was initially too high (around 84 degrees), so I slowly dropped the temperature to 76 degrees over a two day period.
2. I have done three water changes during the week, each for 2 gallons. After reading further, I see that increasing the frequency of water changes doesn't necessarily reduce the amount of water you should take out. I also only added a few drops of prime to the bucket when changing the water, rather than dosing for the entire aquarium (I have since added enough prime for the whole aquarium).
3. There is a presence of brown algae in the aquarium. It covers most of the decorations and the bubbler. I planned on trying to clean this soon.
My readings are 0-0.25 ammonia - 0 nitrites - 5-10 nitrates - 6.6 ph
My cories seem to be fine (they also seem a bit harder to read, but they are moving and none are obviously struggling).
Any help at all would be appreciated. Should I be concerned about a disease? Did I possibly do something that resulted in the fish dying? Is it possible the fish already had something wrong (all three deaths have been from the same "batch" of fish)? Thank you in advance!