What would cause this bleaching?

[Mr31415]

Many factors can contribute. I think I have eliminated most common causes, however I am still baffled why my coral have bleached. Most of the coral I introduced to my 90g reef tank started to bleach anywhere from 1 week to 2 weeks after introduction. In most cases I placed the coral either at the bottom or midway up in the water column.

I had 2 x 250W 14k MH over them, about 20cm from the water column. The MH's are double ended but do have glass cover plates, and I have glass cover plates on the top of the tank - so I doubt this is UVR.

My photo period is 9 hours. Both MH's were turned on at 09:00 and off at 18:00.

Water params are all good (Tested using Salifert/Elos/Apex/Refractometer):

pH 8.43
Temperature 26.2 C
Specific Gravity 1.026
Ammonia - NH3 0 mg/L
Nitrite - NO2- 0.0 mg/L
Nitrate - NO3- 0.2 mg/L
Calcium - Ca 420 mg/L
Alkalinity 8.8 dKH
Phosphate - PO4- 0 mg/L
Magnesium - Mg 1300 ppm
Silica - Si 0.015 mg/L

No major changes are happening in my water parameters. I did started dosing Kent Tech-M Biomagnesium to raise Mg from 1200 to 1300 over a period of a month. Temperature varies between 25.0C and 27.0C depending on the time of day.

All these coral were brown when I introduced them to the tank. My zoanthids are not opening up properly either. My mushrooms seem to be doing great though.

I have good flow in the tank. At the bottom is a recent tank shot. I have a refugium (reverse lit) where I grow cheato. Algae growth is minimal in the tank.

http://www.waldonell.com/images/_53W8117.jpg

http://www.waldonell.com/images/_53W8121.jpg

http://www.waldonell.com/images/_53W8125.jpg

http://www.waldonell.com/images/_53W8130.jpg

Full tank shot from yesterday:
http://www.waldonell.com/me/sites/default/files/images/_53W8165.jpg

I bought a 6 x 54W T5 fixture yesterday with 4 x AquaBlue, one Actinic and one Pure Actinic T5. This replaced the MH fixture, in an attempt to see if this was due to light shock. The more I think about it the less it makes sense, since if lighting was the issue, the polyps would retract/close down, not true? These coral's polyps are all fully open the whole day.

Help/advise would be highly appreciated.

[allnatural]

are you feeding your corals? it seems you have enough light. if the corals are not releasing enough nutrients for the zooxanthalle they wont photosinthisize or give the coral any nutrients. the coral will expell the zoxanthalle or the zooxanthalle will die resulting in bleached coral. try target feeding the corals and see if that helps. just my thoughts.

[tahiriqbal]

What was the reason for your mg to come down on the first place? Are you using any kalkwasser as its known to decrease mg levels and by the way target feed wont do much if something is missing from your water colum!! How old are your MH bulbs? Since you added your T5 only yesterday and bleacing been going on for little while, I would think it could either be your old MH bulbs or as I said mg and calc levels being unstable. I am sure someone else would explain better than me.

[Mr31415]

I do target feed the corals I know accept food like my yellow polyps and some zoas. The rest is fed once a week using a mixture of zooplankton, phytoplankton and cyclopeeze which I throw in the tank and turn off the skimmer, at night.

Could it be my salt? I am using Instant Ocean. That is also the reason (I think) for my low Mg levels. Yes I am using kalkwasser through my topup unit.

MH bulbs are 2 months old. Mg and Ca has been at their current levels for about 2 weeks now. Ca has always been at around 400 and Mg was 1200 -> 1300. Let me retest now.

[Mr31415]

Sorry for replying to an old thread, but I found the source of this issue. The test kit for Mg was faulty, and measured too low. So the Instant Ocean salt had good Mg levels, but the test kit showed about 250mg/L too low. So when I dosed 250mg/L Kent Tech-M Biomagnesium, I raised it to something above 1500mg/L. From past experience when I deliberately did this to kill off Bryopsis, this causes some softies to bleach.

Once I started doing large water changes the corals started to recover. Hope this helps someone someday.

[Sea-nut]

I am glad everything is back to normal. But Mg of 1500 would not hurt corals, as far as I know.

[Mr31415]

Two separate tanks, same dosing of Kent tech m and same results - some softies bleach. It was exactly the same - polyps fully extended but corals were bleached.