First, congratulations on a really nice 13 year old tank! That kind of longevity puts you in the top 10%, if not top 5% or better of all reefers.
I think you have some serious water chemistry issues. First, dump the API test kits. They're OK for fish only but once you get into coral they are simply insufficiently precise. I like Salifert, but Red Sea and Hanna are also very good. Get some new kits and test with those.
Based on what you posted:
If your phosphates are really 0.25 ppm then that's the first place I'd look. They should be below 0.03 ppm. From the photos it looks like you have an algae problem, which would certainly be explained by the high phosphates. That would not be surprising for an old tank with little to no filtration. Eventually, nutrients like phosphate and nitrate are going to add up and monthly dilution from water changes is not likely to keep up over the very long term without some efficient nutrient export. BRS TV has a pretty good video on You Tube about how to control algae. "Eat, Sleep, Reef" has a really good one. Recommend watching both. I also recommend BRS's recent videos on test kits and salt.
At 125.3 ppm, your Alkalinity (Carbonate Hardness) translates into a dKH of 7.02 (most of us on RC measure alk in dKH). That's way too low for corals, which use it to build their skeletons. Recommend SLOWLY raising that to 8.0 - 9.0 dKH (or 143 - 161 ppm). It's also way out of balance with your Calcium, which is very high. Shoot for Ca around 420 - 430 ppm. Higher won't hurt but it will cause lots of precipitation and scaling on pumps, heaters, etc.
You didn't mention Magnesium, which is necessary for maintaining a balanced Alk and Ca ratio. Look there and fix that first. Dosing the others when Mg is below 1300 is a losing battle due to the way the ions interact in the water.
Summary of what I would do in your case:
1. DON'T PANIC! Go slow, corals cannot tolerate rapid changes in their environment even if you are making it better. It takes time for their metabolic processes to adjust to changes in water chemistry.
2. First, take a look at the chemical composition of your salt. If you're buying cheap salt suitable for fish only aquariums you might have to add a lot of Ca, Alk, and Mg to get it up to good parameters, not to mention the trace elements which most of us never even test for. Get a good quality reef salt, then do 3 large water changes of about 50% over a 3 - 5 day period. Don't do it all at once - rapidly increasing Alk can be very hard on some corals (see #1 above).
3. Get new test kits.
4. Test Alk, Ca, Mg, Nitrates, and Phosphates.
5. Consult Randy Holmes Farley's chemistry calculator (Google it) for dosing instructions.
6. Dose slowly to get Mg up to 1300 - 1400 ppm. Don't raise it more than 100 ppm in a day.
7. Raise Alk to 8 - 9 dKH and let Ca fall naturally to 420 - 430 ppm. Do not raise Alk more than 0.1 dKH per day. Slow and steady is incredibly important.
8. Take a hard look at your nutrient export strategy. A cleanup crew is important but insufficient. Your nitrates are OK but You need to get your phosphates under control. Test after your water changes. If it's still high then I recommend some Granulated Ferris Oxide (GFO) to bring it down. You might want to get yourself a $20 reactor and a $15 pump for this. It works best if it can tumble gently. You don't have to run it all the time but if your phosphates are really 0.25 PPM then you need it now.
9. It is likely that your coral have grown to the point where water changes alone are unsufficient for maintaining parameters and you will need to start dosing, if you're not already. For your setup, I would start with Kalkwasser in your topoff. Research it and start slow and dose carefully.
10. Go slow! Corals cannot tolerate rapid changes in their environment.
I strongly recommend second opinions on all this, so other RC members, please chime in. I also recommend research in trusted sources like Randy Holmes Farly, BRS TV, Reef Central's Marine Chemistry Forum, and others.
Good luck and keep us posted!
