I have a danio with a tail that is drooping and some sort of wound that formed on its side about a few days ago. Could it be neon tetra disease? Should I euthanize it? It seems that it has some trouble swimming. Picture below.
12 black neon tetras
4 honey gouramis
2 german blue rams (don't get 2 males)
Make sure that your tank is at least 3 months since the date when you finished cycling the tank before you add the rams.
I would up the number of kuhli loaches to 8-10 and do 3-4 honey gouramis. Both cardinal tetras and neon tetras would work, but personally I would go with cardinals because neon tetras are usually pretty sensitive. Plus, I prefer the appearance of the cardinals over the neons.
The peppered cories need cooler water than the betta, and they need at least 6 of their own species. 78f is also too warm for the peppered cories. They also need at least a 30" long tank, as they can get up to 3" long. I would rehome the cories.
I wouldn't do the neon tetras because they need...
If your tank is heavily planted and/or lightly stocked there's nothing wrong with having zero ppm nitrates. If your tank isn't heavily planted or lightly stocked and you have zero nitrates, then there could be something wrong with your cycle.