If the tank was at 75 and then moved down to 70, that would cause a decrease in activity. Some do also tend to go through phases where they'll be less active for a bit, then go back to zooming around everywhere. Maybe try changing around some different foods and such, but if he's going for the spinach, I'd go with that for a bit. Maybe he got bored with the other options, lol. I'd keep an eye on him for now, but it may not be any cause for concern.