corals dying

I have been having trouble with my LFS's the past couple of weeks. I have been cleaning the tank and refugium and filter out pretty well since I started having issues. My parameters haven't been jumping around, maybe just my Ph.
It is an established tank.

My current parameters
salt= 1.024
PO=0
Nitrates=5.0-8.0
nitrite=0
Amonia= 0-0.25
Ph=7.9

I don't know what happened with my nitrates possibly the dead corals?
ALl the fish and inverts have done well except the sandstar that died.

I am also getting a red residue possibly some sort of fungus/algae. I have attached some pictures of it and what it is doing to my hard brain coral. The left side is bleaching out.

Any feedback would be great! I have been doing weekly 10% changes and looks like I am going to do about 20% to lower my nitrates.

Your nitrates are fine. Nitrates that low wouldn't kill the corals (what brand test kit are you using that measures nitrates at 8?) What kind of lighting do you have? What is your tank's temperature and how much does it vary between day and night?

+1 Biff...

Also, what kind of lighting and flow do you have? Sid you acclimate the corals to your lighting?

I've had the corals for a few months now. They just started taking a turn for the worse.

Flow- I have 2 heads in there. one on each side. can't think of them right now.

Test- I use API.

The temp seems to vary alot. It stays between 75-82 degrees. But I got different readings from the actual thermometer in the water and a digital one before it fell in and the battery got wet :) Don't worry it was only in for a second.

Lighting is a 4 bulb T5 config. I have 2 days and 2 blues. it is a three setting light.
I keep the blues on for an hour to 2. Then switch to day lights for 8+ hours then I shift over to the third setting for about an hour or 2. Am I doing the lighting wrong? The bulbs are about 4 months old.

Also after my clean out and 20% water change last night. Everything seemed to open up quite beautifully. I am concerned about the red algae though as I' had it before it is hard to get rid of. But this stuff looks different then the stuff I had before as it its on the sand and seems a darker red.

It sounds like you are good for flow, and probably okay for lighting (although 4 bulbs for a 75 is on the low side, I don't think that would be the cause of your corals dying). The temperature swings are a concern and from the information you've given, seems to be the most obvious culprit. Anything greater than a 2 degree swing over a 24 hour period can cause corals to suffer. Do you use a heater? If not, I would suggest getting a heater and setting it to 80 or so to keep the temperature in the 80 to 82 range. Or, alternatively, get a clip on fan from Walmart and have it blowing across the surface of the water to lower the temperature from 82. You should have it on the same schedule as your lights.

I've always had trouble maintaining temp. especially this summer with the high temps.

I don't use a heater until winter because I usually have trouble keeping the temp below 82 as it is. It is probably the one aspect of this hobby I have the hardest time controlling.

But temp shouldn't spark algae growth? I thought that had to do with lights and water condition?

Keeping the temperature stable at 82 is much better than having it jump around from 75 to 82. I live in a hotter climate and always kept my tanks at around 82 to 84 because I didn't want a chiller, and that's just how hot they got.

Higher temps can promote algae growth, but you are right -- nitrates and phosphates are ultimately the cause of algae problems.