Tank Temperature Problem

[cardsfan16]

Hi Everyone,
Last week I setup a 75 gallon tank. I have two Rena XP 2s, two Maxijet 1200s, and a ProHeat 250w (maybe 300w) titanium heater. My lighting is the 48" Coralife Lunar Aqualight.
The problem I'm having is the temperature in the tank has been steadily increasing on a daily basis for the past several days. I have the heater set at 76; however, the temp has now crept up to as high as 81.1.
The odd thing is, this temperature increase seems to be happening almost entirely overnight when the lights are off. Two night ago it was in the 77s, but up to the high 79s the next morning. Last night it was at 80.5 when I turned the lights off, but was up to 81.1 this morning when I went to turn on the lights again. I just checked on it a few minutes ago, and the temperature had actually dropped down to 80.6, during the day, with the lights on.
I'm using a digital thermometer, so I picked up a standard one and verified both are reading the same temp. The tank is in our basement, gets no direct sunlight, and is not near any air exchanges. The basement air temp stays just about the same all day and night, which is around 74.
I'm really at a loss to explain this, and was hoping someone might have some ideas. On a related note, I added 50 lbs of cured live rock yesterday (first thing I've added to the tank). At what temperature do I need to start being concerned about its well being?

 

[who dey]

you will be amazed at what a fan blowing across the top of your water can do for your temp. my friend floats 2 liter bottles filled with frozen water in his sump on really hot days, it works pretty will, kind of a half assed chiller!!!

 

[who dey]

mine hovers around 82 during my lights on cycle over the summer and i;ve never had any problems, anything more than that should be cause for concern

 

[cardsfan16]

I can see the temp increasing a bit with the lights on, and it does a little bit. What really has me confused is why it is increasing so much overnight with the lights off. The heater is set as much as 4 or 5 degrees below what I've been seeing the temperature at over the past couple of days. I haven't even seen it activate since earlier this week.
I just can't seem to figure out why it seems to keep creeping up like this with the lights off. I have the temp sensor for the heater and the digital thermometer right next to each other, so they should be reading the same temp. The heater is placed well away from both of them.
Could it be some of the equipment heating up the water? I did just add the two maxijets a few days ago, right before I saw the temp increase begin. They don't seem to put off any heat though.
If it steadies out at 81, I guess I should be OK. I'm just worried about it getting any higher since there doesn't seem to be anything I can do to prevent it.