Acceptable Temperature Swing?
 

How big of a temperature swing is acceptable through the course of a day? I've been cycling my tank with the lights off, but decided to run them for a few hours today and the tank temperature climbed 3 degrees. I wasn't really expecting this as I'm using T5s, and I'm starting to think I may need to add a fan or something...

Apparently there are some studies taking place that monitor the temperature changes on average reefs and they show quite a bit of variability so I believe there not that detrimental. That being said, consistency is #1 in my books for reef care so if you can control it I recommend it. Reefcentral had some good reading on the relationship between day/ light cycles, temp and pH if you were looking for some good info.
Levi
*and my reef swings 3-5*f during summer months.

78-81 my tanks swing in summer

My tank swings 4*f in the summer

I see 1 deggre shift in my tank, using a 1/4 hp chiller though with a aquacontroller JR.
I do find though that the chiller heats up my livingroom a bit too much if i have all the windows closed.
Then it creates a cycle.
The hot room heats the tank the tank needs to be chilled the room gets hotter untill i get too hot and have to open a window.
Hope this winter my windows dont freeze up with moister like it did last year when i was running a cooling fan.

Depends on the corals you keep, some are more sensitive than others. The more steady you can keep it the better. Maintaining you tank temp a little higher @ 80F will typically result in less swing than if maintained at 78F.

If it jumps up 4 degrees once in a while on those hot summer days I don't think it's a big deal but other than that I would aim for less than 2 degrees.