What could be off in my tank affecting coral?

[steallife904]

I have been having an issue lately with my corals receding and dieing off... I cant seem to stop it. It all started about 2 months ago when I slacked a bit on the tank and over dosed ALK (had a baby in October that led to the slacking). I lost about 70% of my coral due to the alk. I slowly got everything back to normal and thought all was well, even dropped in a few frags to see how they would do. Stuff was fine but all of a sudden its like they are dieing off again. Chalice and acans are receding, have a mini Duncan colony that looks like its dropping its heads off 1 by 1, one of the new monti frags I got is receding and tuning white.... most of the sps in there though seem fine?

I have been doing my weekly water changes. I tested phosphate which was undetectable (like always), checked ammonia...0, alk 7.8, calc 450 (alk and calc are close to what they where before alk overdose in October) use to be 7.7-7.8 alk and 420 calc.
I did change out 2 T5 bulbs a month ago but they only run 2 hours, I also cleaned a UV sterilizer quarts sleeve and cleaned my 2 XF150 powerheads. I have increased flow in UV incase I was killing something off and slowed down the pumps incase flow was to high but nothing is helping.

What could be off or an issue causing me to fail all of a sudden with corals? Have never had an issue this bad in the years of this hobby.

 

[steallife904]

also should note fish are all fine. I have checked for stray voltage as well and there is none. No cleaning products around the tank, nobody has been around it to have dumped anything in or anything like that.

 

[steallife904]

also salinity has always been 1.026

 

[GimpyFin]

Do you know what your Nitrate is at?

 

[nuttyd]

and magnesium?
How much water change weekly, how big is the system.

 

[steallife904]

Those are the 2 test I didn't get to this weekend, I will check both today. The system is probably just over 150 gallons. I do a weekly 10 gallon water change. This tank has been up for 2 years. Was doing great prior to alk overdose in October. Most colonys that where in the tank came from the previous tank that ran for 7 years. Had many large sps colonys, acans, favia, few chalice, etc... most of them gone. One birdnest colony I had growing for 10 years. While I know the alk killed them off I am not sure what is going on now. Will check nitrate and mag tonight. Anything else? I also feed the tank heavy with frozen Rods food to feed fish and coral everyday and have done this way for years so I feel pretty good about the coral getting food.

 

[steallife904]

well i checked magnesium.... it was 840. That might be the issue. i will slowly bring it up. i also decided to check the salinity in the tank again, it was 1.027, slightly high. added some extra RO to bring it down close to 1.026. will keep dosing mag and see if that changes anything.

 

[nicholasb]

Mag is very important for coral growth/calcification, not just important for holding calcium and carbonate suspended in the water. At 1200 ppm Duncans, torch, hammer head, and frogspawn start to close up. You need 1,260-1,350ppm. That must be your problem if it is 840.

 

[steallife904]

cant believe I never thought to check the mag. I ran 3 test last night and all came back at 840. That has to be the issue, and I have no idea how long its been off. Looks like I will need to start dosing mag.

 

[snorvich]

Ca, alk, mg test weekly,