Sunny day in the Caribbean and your on vacation. You step into the beautiful crystal blue water and wow...it's so comfortable, it's almost like bathwater. Yet..there by our feet are the very animals we keep in our tank.
OK, so, what temperature should you keep your tank at? As you would expect, this is a hotly debated topic, similarly to where your tank's specific gravity should be. Of course, it all depends on what you want to keep in your tank. General ranges are 75-89 F, but, make sure you choose a temp that will keep your inhabitants healthy. Margarita snails and various gobies, eg, prefer cooler water temps.
Principles to keep in mind:
1) choose a tank temperature that is consistent for the environment that your inhabitants thrive in, i.e., research your animals'/inverts' needs
2) steady temperature set point is preferable to large swings of temperatures in normal ranges; eg, I'd rather have a steady temp of 83 degrees than daily temp swings between 78 and 82. This can cause stress
3) Marine tanks are not cheap. Invest in a reliable heater; if you run hot, know how to keep your tank cool (eg, chillers, hood fans, external fans, more efficient pumps, etc.)
4) Use a backup thermometer. Glass or LCD's are adequate backup, eg, to a digital thermometer. Use whatever gives you steady, reliable and consistent temps.
5) Use a cheap heater to bring your sea water changes up to tank temp before dumping it in
6) Adding additional pumps. leaving the hood open, upgrading your lighting, all may effect tank temp
What do I do? I try to maintain my tank at 'ocean parameters'. For me, I have accepted that as 82-83 degrees and s.g. of 1.026. My digital temp probe averages 81.5-82.7 degrees and am comfortable with tank temps at 83.
Nano nano. SH
OK, so, what temperature should you keep your tank at? As you would expect, this is a hotly debated topic, similarly to where your tank's specific gravity should be. Of course, it all depends on what you want to keep in your tank. General ranges are 75-89 F, but, make sure you choose a temp that will keep your inhabitants healthy. Margarita snails and various gobies, eg, prefer cooler water temps.
Principles to keep in mind:
1) choose a tank temperature that is consistent for the environment that your inhabitants thrive in, i.e., research your animals'/inverts' needs
2) steady temperature set point is preferable to large swings of temperatures in normal ranges; eg, I'd rather have a steady temp of 83 degrees than daily temp swings between 78 and 82. This can cause stress
3) Marine tanks are not cheap. Invest in a reliable heater; if you run hot, know how to keep your tank cool (eg, chillers, hood fans, external fans, more efficient pumps, etc.)
4) Use a backup thermometer. Glass or LCD's are adequate backup, eg, to a digital thermometer. Use whatever gives you steady, reliable and consistent temps.
5) Use a cheap heater to bring your sea water changes up to tank temp before dumping it in
6) Adding additional pumps. leaving the hood open, upgrading your lighting, all may effect tank temp
What do I do? I try to maintain my tank at 'ocean parameters'. For me, I have accepted that as 82-83 degrees and s.g. of 1.026. My digital temp probe averages 81.5-82.7 degrees and am comfortable with tank temps at 83.
Nano nano. SH
. I've been on shallow water dives (~30 ft) like you and havent bothered with a wetsuit at all. My uncle (florida native) is a wuss though and still wears one 
Back to my point though, I have noticed that on deeper dives especially, where more hard-coral varieties (acros, montis, etc) grow the temp is typically 75 or less, even in mid-summer (60-150ft deep). That thermocline gets tricky.
