How low can you go with Reef tank temperatures?

[ThRoewer]

What is the lowest possible water temperature for a tropical reef tank?
Right now I keep my tropical reef tank between 24 and 27 °C.

I looked into lowering it further when I planned to add a group of ventralis anthias, though Capsalid Monogeneans scrapped that plan. For now I'm looking into it to possibly lower energy cost and just in case I try the ventralis again.

I've seen post where people kept tropical reef fish in the 20 °C (68 °F) to 24 °C (75 °F) temperature window of the warm temperate zone. Fish that have a wide depth range are likely more tolerant of lower temperatures, but I could see issues with shallow water species like clownfish (A. percula), shallow water anemones (S. gigantea) and of course corals.

So, how low can you go without running into problems?

 

[ReefsandGeeks]

I'm not sure how low you can go without problems, but one concern I'd have with pushing the boundries would be how long your tank could last is power was lost. Lets say 70F is the lowest you can go. If you keep your tank at 78F and lose power, your tank can drop 8 degrees before issues relating to temperature start. If your tank is already at 70F to start, than you'll notice issues very quickly when the power eventually goes out.

If looking to save money, you could look into insulating any part of the tank that isn't for viewing. You can get foam sheets made for insulating house walls and cut it to size, then tape or add a bit of glue and stick it to the back, and maybe a side or two of your display, and all 4 sides of your sump.

Actually, looking at your setup, it looks like you have a multiple tank system, which means you do have a lot of surface area for heatloss.

 

[Moort82]

Fish wise I think temperature is less important but corals are more finicky. I knew someone who went on holiday for a month. Two days before they left they did a water change and unplugged the heater. When they got back the fish were fine but the corals looked a little annoyed they then discovered they didn't plug the heater back in. The tank sat at a room temperature which was 17-18c.
I've heard similar stories several times were heaters failed so they can accept it if only for short periods. I have also read but can't remember where that under 23c corals have a hard time calcifying, so if growth is important then that might set your limit. I myself have a 23.5-24c winter minimum and in summer it runs a couple of degrees higher for free.