Here is just a warning, I hope you can learn from my mistake... no, not the cuttles, my other tank. I have a 30 gallon that my cuttles grew up in - since they weren't using it, I put a little Percula clown, a yellow watchman goby and a skunk shrimp. I haven't been paying as much attention to this tank as I should have been. The thermometer broke, and I never replaced it, I just figured that the heater was working (it was new), and it would maintain a constant temperature. I top off with fresh RO/DI water when needed, feed occasionally (my four-year-old does most of the feeding) and clean the filter. Well, today I came down to the basement and noticed the watchman goby out in the front of the tank. She is usually very secretive, so at first I thought, "cool, I can see her," then I noticed she was lying on her side and I thought "yikes, she's dead!" Then I notice that the clownfish is also on the bottom of the tank on its side. So I rush to do a water change, thinking the water parameters must be off. First rule of salt water aquariums, always have premade salt water on hand for emergencies... I had about 5 gallons already made, but I was hoping to do a 25% change, so I didn't have enough. But, I had just changed the cuttles water, but had not yet dumped the old water, so I used that water, thinking if it didn't hurt the cuttles, it had to be better than what the fish were in right now. Then I stuck my hand in the tank and realized how COLD the water was. My heater had stopped working and the water was unheated, probably for at least 12 hours - since when I checked the temperature it was 57 F - yes, that's right 57 F! No wonder they were lying on their sides... and the shrimp had stopped moving. Second rule of aquarium keeping - check on your tank every day!!! And keep extra equipment on hand. I grabbed my extra heater, that I use to heat my premade salt water before I do water changes, and put it in the tank. I knew that it would not heat the tank fast enough to save the fish, so I took about 1/2 gallon out, put it in a glass bowl and heated it in the microwave, then dumped it back in the tank. I kept doing this until I got to 68 F - I raised it about 0.5 degrees every 5 minutes. Then I raced to the pet store and bought another heater. At 68 F. the goby perked up and was sitting in the back of the tank glaring at me and the shrimp started moving again at 68 F. However, the clownfish was under a rock, breathing hard with its tail curled up. I thought it was too late. However, when I finally got the tank to 75 F, he came out and started swimming around. I realize these fish may still die from the shock, but hopefully I saved them this time. I am an experienced aquarist, but I just got too busy with other things. So hopefully this will help someone else. At least it wasn't as bad as the time my professor turned the fan off on the chiller and created an "ice age" in an aquarium...but that's another story!