Here are the specs on my tank: 15 gallon tank, heated to 76, filtered. Live plants, gravel substrate. 25% water changes once a week (conditioned, plants fertilized).
Nitrate: 40 ( I know this is a little high, I'm working on keeping it lower)
moderately hard water
Stock: 4 zebra danios, 4 false juli cories, 1 male sunburst platy, 1 tiger nirite snail. They have all been living happily in the tank together for 6 months, no problems that I'm aware of.
One of my danios, the smallest one, is acting weird. His dorsal fin has been down, he's had no appetite for 2 days, he's been "hiding" a lot at the bottom and staying away from the others, and he will often stay very still except for what looks to be involntary pulsating, like a little fish seizure.
There's no outward sign of disease that I can see, and I've never seen a fish shake like that involuntarily.
Any thoughts on what this could be? None of the other fish are acting strangely at all, and this guy just started yesterday.