Heat Fluxuation Issues

[Blackadder]

I have a 13 gallon nano reef tank holding about 10 gallons with a couple fish and my 'reef'. I have a 50 watt heater, but I'm having a tough time maintaining a consistent water temperature. I recently replaced the stock lighting hood for a more advanced lighting system because it was clear the stock lighting was heating my water during the day. Now, with what I believe is a pretty decent heater and all 'known' factors removed, the temperature still drops at night. I would like to keep the temp around 80, but the temp drops to around 76.5 and it takes all day to get the temperature up to a more appropriate level. Nothing happens at night that doesn't happen during the day! That bothers me because I believe it is stressing the fish and my three little corals.

Is a 50 watt too small?

 

[AfaceInTheCrowd]

Don’t assume the thermostat on the heater is accurate

 

[Blackadder]

Don’t assume the thermostat on the heater is accurate

I've been tweaking it up bit by bit. It seems to want to 'reach' the set temperature, but the temperature drops every night. I think it's a pretty good heater, but it doesn't seem to be doing it's job of maintaining a stable temp.