Few Corals Acting Funny, Need Help Finding Culprit

Ok so my tank is acting a little strange lately. Recently some of my corals have either shrank in size or died completely and I cant figure out what the deal is. The tank is a 40g and has been up and running for years. Recently checked the levels and came up with this:

Temp - 78 degrees
PH- 8.3
Nitrate - 0
Nitrite - 0
Calcium - 460
Salinity - 1.025
Phosphate - .5ppm (has started creeping up recently due to my mom overfeeding them)

Equipment is:
4x65 PC lighting
Modded Sea Clone 150 Skimmer
Ehiem 2236 cannister
Maxi Jet 1200 powerhead
Hydor 300W heater

The corals in question are: Xenias that were alive and huge and pretty much vanished completely in about a weeks time, clove polyps that were expanding pretty well but have decreased in size and numbers, and also some Palys that just seem to be a little more closed that usual. On the flip side I have two fuzzy mushroom farms that are doing excellent as well as a Yuma that is huge and growing by the day. All of the fish and inverts are very healthy and nothing has changed at all equipment wise. I've even got a very nice colony of pods throughout. I dont have any fish or inverts that would bother the coral and havent added any new tank mates in a very long time. Is there anything I should be testing for or is there something in there I should be looking for like worms, etc.? Dont really know what many diseases look like so I could never identify them without a pic.

 

Xenias are finicky in my experience.

I would make sure your lighting(bulbs) is still good.
may be possible your nitrates are a little too low for the softies in question.
phosphate is probably too high.
iodine may be low(but I am weary of dosing that).

 

Xenias usually do like a "dirty" tank with some nitrates, that could be your problem with those. As far as the cloves, mine haven't really grown either, same with my palys. I would suggest checking the bulbs for an upgrade or what about your Mg of Alk levels?

 

The bulbs are less than 2 months old so I know they are perfectly fine. As for alk, I honestly havent checked it lately but as of last week things were fine in that department. The Nitrate levels have been at zero since the tank was cycled and have remained that way so I'm not sure that would be an issue either. The phosphate levels are the only thing out of the ordinary but would that little of a difference really cause things to shrivel up and die?

 

No new additions that could have brought in some kind of pests?

 

i think he might mean your NO3 is too low. Softies in gerneral need a little gunk in the water to be super happy. I thought alk too, get a test you wont regret it.

 

Nothing at all, we havent added anything since February. The No3 level has been 0 for years and everything in there was always fine, thats what confused me. Nothing at all has changed and the corals have been in there for at least a year and were thriving until just recently. I'll grab an Alk test kit next time I go to the store and see if maybe that went south on me. Also do ya'll think a large water change will rid the tank of phosphates or should I toss some media in the filter?

 

what kind of test kit do you test nitrate with? If its a midrange one, zero could have been few ppm. Bacteria tend to use both phosphate and nitrate in combonation, raising phosphate could indicate running out of nitrate.
personally I wouldn't rule that possibility out but it wouldn't be my first choice of the problem

 

Make sure your sand hasn't gone toxic with hydrogen sulfide. This usually shows with an increase in nuisance algae though. The increase in food source could be the cause also.