I have central AC. Runs all year round. No fan option in that room. Problem is sump/skimmer sits inside a cabinet and I feel air is not circulating well in there.
A cabinet well-sealed enough to trap CO2 should also trap heat just as well.
This is too easy not to test:
Just as a test, leave the door open to the cabinet - maybe also setting up a temporary fan to blow into the open door. Any fan will work. (Don't worry if you don't have one...just give it a few hours to work passively.) This might make a difference if it's as well-sealed you're imagining. If it does then you know an easy modification to the cabinet is a sure fix. Two holes - one down low for a fan to suck room air from about floor level into the cabinet, a second hole on the opposite side of the cabinet up high to let the fan push hotter air out of the cabinet. It might even work well enough without the fan.
If you want to get technical, hang a thermometer inside the cabinet with the door closed before you do anything and see what kind of temperatures you are getting down there. You'll also be able to tell very quickly whether your test is working or not.
SPS have been losing color. Some are 85% gone. Then not all SPS have been affected. Also have some powdery brown algae growing on my monti caps. Got some cyano too. Some other montis have green algae. I started with a BP reactor 2+ weeks back to combat high NO/PO that I've had for a while. Some of the monti problems (brown powder algae) were there before I started BPs. My BPs reactor is a Reef Dynamics which allows one to fine tune the effluent, which has been on a slow trickle.
These are not good signs. Did you ever determine (and preferably eliminate) the cause of your nutrient issues?
I'd be poking around in the sump/filters, sand bed and in/around the rocks looking for detritus.
Use a powerhead to blow out the rocks at least once to see what you get. If you get much, improve your flow to prevent it from happening as quickly next time and do this blowout regularly for a few weeks to clear things out.
Swizzle your finger or a stick around in the sand at strategic places - bases of rocks; underneath powerheads or return outlets; etc. If you see much, I wouldn't hesitate in starting to remove the sand bed. Begin with the worst areas and remove no more than 1/4 of the bed at a time, and I'd wait a week between sessions - allow the bacteria in the rocks (and your pellet system) to grow and stay ahead of the demand as you remove large amounts of surface area from the tank in the process.
I can bet my CO2 is high. Someone else mentioned the 2 Little Fishies CDX adaptor. Will probably try that since there is no way I can get the airline out of the house.
Simply getting the airline out of the cabinet may be enough. Again try the door open+fan test above before you do anything more involved.
If none of this helps, then you have some idea that it might actually be the house air that is the problem, possibly warranting a CO2 scrubber and/or other measures.
Good luck!
-Matt