I purchased the guppies today, and fed them a little bit, since i don't know how much the store fed them. When it first introduced them to the tank, the all gravitated to the top, and i thought it was because they were still hungry, so i fed them a bit more, a few teaspoons of flakes, ground up between fingers, and they started exploring. when I came back up, later this evening, i found 2 of the 3 swimming at the top, like before, but near the heater. is this because the water is too cold? and one is all the way down at the substrate, moving back and forth very rapidly, almost like he wanted to dig a hole in the substrate. is this normal too?