I'll back that up, normally the water is 35 degrees at 8 am and what's left by 3 pm is still just over 20 degrees - tank not insulated but unlike a bath is still enclosed - water takes a long time to cool down when inside a container and we are talking a decent volume of the stuff.
Immersions are a great way to heat your tank, this week I finished upgrading the vans from the portable travel ones to a permanent fixed version.
So what are you using Duncan

?
Darran