Amano Shrimp On Heater

[Bugdozer]

I'm possibly being a little nervous here because I've read so many articles about how amanos will drop dead at the slightest thing, but for some reason today mine have been clustering at the top end of the heater. As far as I can see there's not a lot of algae for them to pick off there, but it is near both the surface of the tank and right near the inflow from the filter, hence it's probably one of the most oxygen-rich areas of the tank. My fish are behaving normally. Could this be a sign that something's wrong, or do they just like heat, or fancied all clubbing together up there for a change?