Bad Water Change Causing Corals To Die

[reef tank journal]

Hi reefers,

I'm a fairly new reefer. Been in the hobby for almost a year. A couple of weeks ago I did a 10 gallon water change in my 69 gallon AIO tank and it went all downhill from there. 1 chalice coral and 3 torch coral have died and I'm trying to prevent anymore from following. I did a water test yesterday in my tank and noticed my magnesium was low (1093) and I'm pretty sure that's the main culprit.

My tank was thriving before the water change. :confused-face: To be honest, I'm not sure why I did a water change. I'm on team no water change, but I keep feeling like I'm doing something wrong by not doing water changes. Reading and seeing the necessities of water changes just convinced me to do this water change. The last time I did a water change was in Summer of 2024.

My questions for you all is, do you think my coral died because of the low magnesium? Did I just use a bad batch of reef crystals mix? (I'm almost at the bottom of the bucket) How can I prevent this again? Should I test the new water before actually doing the water change?

I guess I'm just a little bummed out by losing some coral and feeling like I failed my tank.

 

[vetteguy53081]

Hi reefers,

I'm a fairly new reefer. Been in the hobby for almost a year. A couple of weeks ago I did a 10 gallon water change in my 69 gallon AIO tank and it went all downhill from there. 1 chalice coral and 3 torch coral have died and I'm trying to prevent anymore from following. I did a water test yesterday in my tank and noticed my magnesium was low (1093) and I'm pretty sure that's the main culprit.

My tank was thriving before the water change. :confused-face: To be honest, I'm not sure why I did a water change. I'm on team no water change, but I keep feeling like I'm doing something wrong by not doing water changes. Reading and seeing the necessities of water changes just convinced me to do this water change. The last time I did a water change was in Summer of 2024.

My questions for you all is, do you think my coral died because of the low magnesium? Did I just use a bad batch of reef crystals mix? (I'm almost at the bottom of the bucket) How can I prevent this again? Should I test the new water before actually doing the water change?

I guess I'm just a little bummed out by losing some coral and feeling like I failed my tank.

Although below the recommended threshold( 1300-1400), it likely was not culprit. Did you use same temperature water and salinity?
Any change in brand of salt?
What is alk and calcium measurements ?

 

[brandon429]

No that's not a killer

Post tank pic. Nothing you can measure with hobby kits will be easy to pin point, a Pic is far better. When you pour in does it kick up waste?

From the sand?

Pics also show lighting. New tanks tend to chase brighter is better, sometimes reducing intensity stops loss. Pics, tests aren't helpful. Long term issues are most likely, not something you can test other than salinity... you didn't add messed up water correct? You're sure on salt levels/ accurate?

Pics will show if applicable other growing corals we can discern water quality from, if all of them look bad it may be different.

 

[JTP424]

I'd also be curious as to current tank parameters, vs what you used for your water change, if you're "anti-water change" (or against at least :) no harsh feelings!) do you typically keep RO/Salt ready?

 

[Randy Holmes-Farley Verified Community Expert]

I did a water test yesterday in my tank and noticed my magnesium was low (1093) and I'm pretty sure that's the main culprit.

I think it more likely than not that either the salinity is low or the magnesium test is in error, both of which are very common. Actual magnesium that low at 35 ppt is hard to attain. It takes a lot of alk and calcium dosing over a long time to deplete it that much. But if you have been doing that, it may be real.

 

[reef tank journal]

Although below the recommended threshold( 1300-1400), it likely was not culprit. Did you use same temperature water and salinity?
Any change in brand of salt?
What is alk and calcium measurements ?

The water was the same salinty and temperature and I used the same salt.

ALK: 180
CA: 535

I know my calcium is high, but I'm dosing all for reef and I just lowered the dosage so that it can bring it down a bit.

 

[reef tank journal]

I think it more likely than not that either the salinity is low or the magnesium test is in error, both of which are very common. Actual magnesium that low at 35 ppt is hard to attain. It takes a lot of alk and calcium dosing over a long time to deplete it that much. But if you have been doing that, it may be real.

I'm dosing all for reef and the calcium is a little high at 535.

 

[reef tank journal]

No that's not a killer

Post tank pic. Nothing you can measure with hobby kits will be easy to pin point, a Pic is far better. When you pour in does it kick up waste?

From the sand?

Pics also show lighting. New tanks tend to chase brighter is better, sometimes reducing intensity stops loss. Pics, tests aren't helpful. Long term issues are most likely, not something you can test other than salinity... you didn't add messed up water correct? You're sure on salt levels/ accurate?

Pics will show if applicable other growing corals we can discern water quality from, if all of them look bad it may be different.

I'll post some pictures later when I get home of the other coral, and the tank. But the salinity is 35 ppt. I calibrate using the calibration fluid for refractometers.

 

[reef tank journal]

I'd also be curious as to current tank parameters, vs what you used for your water change, if you're "anti-water change" (or against at least :) no harsh feelings!) do you typically keep RO/Salt ready?

I have a RO/DI and use an ATO to maintain the water levels. I don't keep salt water ready though. Whenever I think I have to do a water change that's when I'll prep the water and then do the water change the next day. I do lean heavily on the "anti-water change" because if I see my tank is doing good, my corals are happy, fish are happy, and my tests are coming out good then why mess with it? That's kind of why I am scared of doing water changes.

 

[Randy Holmes-Farley Verified Community Expert]

Salinity is the same as what? What do you think it is and how is it measured?

My expectation is that the magnesium test is not accurate given that additional info (especially the calcium), but it may be low salinity too.

 

[reef tank journal]

Salinity is the same as what? What do you think it is and how is it measured?

My expectation is that the magnesium test is not accurate given that additional info (especially the calcium), but it may be low salinity too.

I try to keep my salinity at 35ppt at all times. I use a refractometer to test it and calibrate it before testing every time.

I also used an api aquaspin tester to get my results.

 

[vetteguy53081]

The water was the same salinty and temperature and I used the same salt.

ALK: 180
CA: 535

I know my calcium is high, but I'm dosing all for reef and I just lowered the dosage so that it can bring it down a bit.

High CA may be your issue as it will affect alk and even Ph and recheck salinity at LFS

 

[Randy Holmes-Farley Verified Community Expert]

I try to keep my salinity at 35ppt at all times. I use a refractometer to test it and calibrate it before testing every time.

I also used an api aquaspin tester to get my results.

Some other folks have magnesium measurement issues by aquaspin being way different than test kits such as Salifert.

 

[BryanM]

fwiw, my calc at home tests are high, and I dose AFR. But my very recent ICP test result showed 462.

its become my understanding that both calc and mag are hard to test accurately at home.

 

[reef tank journal]

Some other folks have magnesium measurement issues by aquaspin being way different than test kits such as Salifert.

I’ll test again using my Hanna tester kit to see if it’s the similar. Thanks!

 

[reef tank journal]

fwiw, my calc at home tests are high, and I dose AFR. But my very recent ICP test result showed 462.

its become my understanding that both calc and mag are hard to test accurately at home.

Might be time to try an ICP test.

 

[reef tank journal]

I'll post some pictures later when I get home of the other coral, and the tank. But the salinity is 35 ppt. I calibrate using the calibration fluid for refractometers.

Not sure if this helps much, but the lights turned on about 15 minutes ago and everything is starting to open up. I still haven’t taken out the dead torch on top of the shelf rock.

 

[brandon429]

Which corals are not affected

Is there total loss pending or select

 

[brandon429]

Whiskers of a lysmata shrimp clue
Top sensitive animal we keep this side of basket stars. Bioindicator of not much wrong in the water he'd be first to go. Not that ions out of whack would kill him, some high or low alk won't, but combined with too bright lighting alk spikes will bleach corals. With lower light it less often does, so we start with that easy hopeful change which doesn't harm any reef to try.

You can rule out water contamination. There's no poison nor bacterial issues, that's a normal reef. The select losses are part of the game. If you begin reacting to test kit levels involving phosphate and or nitrate, nutrient pathways, you'll give that system dinos then really be in a problem.

Reacting to magnesium isn't going to hurt much though. It's not something I've ever needed to assess in a thousand reef tanks worked with those clues above.

You can now rule out bad water change as a cause, it's a coincidence. Reducing light power -15% and sustaining a month will not hurt any system on this board, and for select coral issues bearing those clues above it's a safe and wise thing to do. Ramp it back up slowly afterwards, or keep it slightly less depending on which corals you identify as the challenge ones.

That's not a crashing tank, a few corals declining among majority not declining or bleaching means guessing is all you have left. You cannot harm a reef with quality water changes, feed detailing, and lesser light which is why that's my recommend except for cases where the pic shows serious decline

That above isn't.

Increase/ begin target feeding the corals in challenge. Do physical things different from the steady state that caused some challenge, be working with less light and more target quality feed such as roti feast refrigerated feed, that's quality.

This isn't a chemistry or poison issue, full bet per Pic assessment #13,533.

 

[reef tank journal]

Whiskers of a lysmata shrimp clue

You can rule out water contamination. There's no poison nor bacterial issues, that's a normal reef. The select losses are part of the game. If you begin reacting to test kit levels involving phosphate and or nitrate, nutrient pathways, you'll give that system dinos then really be in a problem.

Reacting to magnesium isn't going to hurt much though. It's not something I've ever needed to assess in a thousand reef tanks worked with those clues above.

You can now rule out bad water change as a cause, it's a coincidence. Reducing light power -15% and sustaining a month will not hurt any system on this board, and for select coral issues bearing those clues above it's a safe and wise thing to do. Ramp it back up slowly afterwards, or keep it slightly less depending on which corals you identify as the challenge ones.

That's not a crashing tank, a few corals declining among majority not declining or bleaching means guessing is all you have left. You cannot harm a reef with quality water changes, feed detailing, and lesser light which is why that's my recommend except for cases where the pic shows serious decline

That above isn't.

Increase/ begin target feeding the corals in challenge. Do physical things different from the steady state that caused some challenge, be working with less light and more target quality feed such as roti feast refrigerated feed, that's quality.

This isn't a chemistry or poison issue, full bet per Pic assessment #13,533.

Thank you for that! I guess losing corals is just part of the game like you said. I’ll keep monitoring the coral and seeing if any looks like they may be in decline and try to help them out. Thanks for the reassurance, I really thought the water change was going to ruin my tank.