Low alkalinity, but pH, Ca, and Mg in range? Please help!

[simplyaquariums]

Hi all,

I have 2 mixed reef systems, which have been running since November 2025 and March 2025 respectively. Up to this point, I have been using RODI and saltwater purchased from my LFS, which sources natural sea water as live in a coastal city in Florida.

I purchased both RODI and saltwater last week and did my water changes on Saturday. When I tested my water yesterday, I was surprised to see my alkalinity had dropped significantly. Usually, alk would be around 7.8 after a water change which was on the low end of the range but consistent for my tanks,

Biocube 29 + Sump w/ chaeto refugium - I have noticed a green yuma mushroom and my gonis are not puffy/extended

Parameters as of 6/29
Salinity 1.027
pH 8.3
alkalinity 6.7
nitrate 8.3
phosphate 0.44
Ca 430
Mg 1400

Waterbox Clear Mini 6 + HOB - I have noticed some of my mushrooms are not fully open (red)

Parameters as of 6/29
Salinity 1.026
pH 8.4
alkalinity 6.7
nitrate (could not test because the reagent packets were not sealed, unuseable)
phosphate 0.24
Ca 450
Mg 1410

I tested the saltwater that I had purchased and measured salinity as 1.027 and alkalinity as 6.8, which is alarmingly low. I understand that a low alkalinity means my tank is more prone to wild pH swings because of the lowered buffering capacity of the saltwater, but what can I do now if anything? I have baking soda and All-for-Reef on hand, but have never used. Any advice is greatly appreciated. Please let me know if I can provide any further information.

Salinity is tested with Milwaukee Refractometer.
pH, alkalinity, nitrate, and phosphate are tested with Hanna Checkers.
Ca & Mg are tested with Salifert titration test kits.

I do want to make my own saltwater, but am a little lost and overwhelmed at selecting an RODI system and salt mix. Thank you, if you read this far.

 

[Hardy_Reef]

I'm not qualified to get into the complex chemistry like @Randy Holmes-Farley, but I will start off by saying the alkalinity of NSW is closer to what you're seeing in this system as opposed to synthetic salt mixes that mix closer to the 8-9 dKH range. At this point, do you supplement with anything or have you solely relied on water changes for replenishment? How long were the alkalinity tests conducted apart? Finally (and it's a matter for some debate) your phosphate levels seem high; Usually 0.05-0.1 is the target (though I personally prefer 0.1-0.15). Sorry for the lengthy response. Hopefully we can get you squared away.

 

[tritonpower]

I would confirm the alk with a salifert. I stopped using hanna for everything but phos due to inconsistency. I found alk was more consistent early on but would become less so over the weeks. Always have a salifert on hand to double check.

I ended up double checking so often I just went with salifert alone.

 

[simplyaquariums]

I'm not qualified to get into the complex chemistry like @Randy Holmes-Farley, but I will start off by saying the alkalinity of NSW is closer to what you're seeing in this system as opposed to synthetic salt mixes that mix closer to the 8-9 dKH range. At this point, do you supplement with anything or have you solely relied on water changes for replenishment? How long were the alkalinity tests conducted apart? Finally (and it's a matter for some debate) your phosphate levels seem high; Usually 0.05-0.1 is the target (though I personally prefer 0.1-0.15). Sorry for the lengthy response. Hopefully we can get you squared away.

I have not supplemented or dosed anything up to this point and have primarily relied on weekly water changes to keep my alkalinity at 7.8. However, due to the sudden drop, I was worried that this was an emergency that needed to be corrected immediately. I had tested my alk in a previous test on 6/15 where both systems were between 7.7 -7.8 dKH. Absolutely, I am aware of my high phosphates. It has been a constant in my Biocube where they initially rose to about 0.55 at the height and have been very slowly creeping down over time, there was a period of time where I was at 0 nitrate and I’ve slowly increased that through feeding and slightly less rigorous tank cleaning.

 

[simplyaquariums]

I would confirm the alk with a salifert. I stopped using hanna for everything but phos due to inconsistency. I found alk was more consistent early on but would become less so over the weeks. Always have a salifert on hand to double check.

I ended up double checking so often I just went with salifert alone.

Ordering it right now.

 

[Hardy_Reef]

I'm not qualified to get into the complex chemistry like @Randy Holmes-Farley, but I will start off by saying the alkalinity of NSW is closer to what you're seeing in this system as opposed to synthetic salt mixes that mix closer to the 8-9 dKH range. At this point, do you supplement with anything or have you solely relied on water changes for replenishment? How long were the alkalinity tests conducted apart? Finally (and it's a matter for some debate) your phosphate levels seem high; Usually 0.05-0.1 is the target (though I personally prefer 0.1-0.15). Sorry for the lengthy response. Hopefully we can get you squared away.

I have not supplemented or dosed anything up to this point and have primarily relied on weekly water changes to keep my alkalinity at 7.8. However, due to the sudden drop, I was worried that this was an emergency that needed to be corrected immediately. I had tested my alk in a previous test on 6/15 where both systems were between 7.7 -7.8 dKH. Absolutely, I am aware of my high phosphates. It has been a constant in my Biocube where they initially rose to about 0.55 at the height and have been very slowly creeping down over time, there was a period of time where I was at 0 nitrate and I’ve slowly increased that through feeding and slightly less rigorous tank cleaning.

Okay, no worries. I'm a fan of double-checking the alkalinity readings with another kit and supplementing with buffer if necessary. 6.7 is low by hobby standards, but you aren't completely in the danger zone. I would watch the phosphate and slowly work it down while making sure nitrates don't bottom out.

 

[00W]

First welcome to R2R!
Tanks look very nice.
Parameters are not too far off so I wouldn't go making drastic changes/adjustments.
I'm going to tell you to start making your own water but you probably know that already.
I've been using an ro buddie for years.
Easy, awesome and cheap.
Use a salt close to your parameters, reef crystals might be a good choice for a few changes.
Nothing wrong with hanna for alk testing I've never had an issue with this tester.
Occasionally I'll add just a liquid alk supplement if my alk goes below 10, which it varies from 8-11 on it's own.
This will raise ph for a bit but it comes back down.
Like I said, just go slow, parameters look really just fine, don't do anything drastic, welcome to R2R and keep asking questions.
Joel

 

[simplyaquariums]

I'm not qualified to get into the complex chemistry like @Randy Holmes-Farley, but I will start off by saying the alkalinity of NSW is closer to what you're seeing in this system as opposed to synthetic salt mixes that mix closer to the 8-9 dKH range. At this point, do you supplement with anything or have you solely relied on water changes for replenishment? How long were the alkalinity tests conducted apart? Finally (and it's a matter for some debate) your phosphate levels seem high; Usually 0.05-0.1 is the target (though I personally prefer 0.1-0.15). Sorry for the lengthy response. Hopefully we can get you squared away.

I have not supplemented or dosed anything up to this point and have primarily relied on weekly water changes to keep my alkalinity at 7.8. However, due to the sudden drop, I was worried that this was an emergency that needed to be corrected immediately. I had tested my alk in a previous test on 6/15 where both systems were between 7.7 -7.8 dKH. Absolutely, I am aware of my high phosphates. It has been a constant in my Biocube where they initially rose to about 0.55 at the height and have been very slowly creeping down over time, there was a period of time where I was at 0 nitrate and I’ve slowly increased that through feeding and slightly less rigorous tank cleaning.

Okay, no worries. I'm a fan of double-checking the alkalinity readings with another kit and supplementing with buffer if necessary. 6.7 is low by hobby standards, but you aren't completely in the danger zone. I would watch the phosphate and slowly work it down while making sure nitrates don't bottom out.

In terms of buffer, could that be baking soda or all-for-reef?

 

[simplyaquariums]

First welcome to R2R!
Tanks look very nice.
Parameters are not too far off so I wouldn't go making drastic changes/adjustments.
I'm going to tell you to start making your own water but you probably know that already.
I've been using an ro buddie for years.
Easy, awesome and cheap.
Use a salt close to your parameters, reef crystals might be a good choice for a few changes.
Nothing wrong with hanna for alk testing I've never had an issue with this tester.
Occasionally I'll add just a liquid alk supplement if my alk goes below 10, which it varies from 8-11 on it's own.
This will raise ph for a bit but it comes back down.
Like I said, just go slow, parameters look really just fine, don't do anything drastic, welcome to R2R and keep asking questions.
Joel

Thank you! I’m happy to be here instead of just spying on all the posts from afar lol. I’m glad to know I’m not too far off, it was just a little scary for me to see such a change from my normal parameters. I definitely want to start making my own saltwater & RO water too. Do you have to use any special add-ons to the RO buddy? I was also looking at the BRS options but there are A LOT of options. trying to wrap my head around everything

 

[00W]

Baking soda works fine, will lower ph for a pinch.
Kent/coralife, seachem both make marine buffer, supposedly keeps ph at 8.3 and will raise alk.
It does, but it's not a miracle or anything like that and contains high amounts of borate salts.
That's really what it is.

 

[00W]

Thank you! I’m happy to be here instead of just spying on all the posts from afar lol. I’m glad to know I’m not too far off, it was just a little scary for me to see such a change from my normal parameters. I definitely want to start making my own saltwater & RO water too. Do you have to use any special add-ons to the RO buddy? I was also looking at the BRS options but there are A LOT of options. trying to wrap my head around everything

Yes.
Too much information.
Way to much.
Ro buddie is easy, no extra parts needed.
It comes completely ready to go, just have to put it together.
I run mine off my washing machine line.
You can buy a tds meter for it for another 20 bucks.
My whole setup cost me about $125.

 

[Randy Holmes-Farley Verified Community Expert]

I think it unlikely that an alk of 6.7 dKH is any concern (since corals thrive in the ocean at similar alk) or a cause of the coral issues you report, but boosting it is easy and a fine plan.

Baking soda works for alk, but AFR will also add some trace element sin case that is a factor for the goniopora and mushroom.

I'll note that if you want to use natural seawater (which is fine) then you may need to dose alk up each time, even if there is no alk consumption in the tank.

 

[simplyaquariums]

Use a salt close to your parameters, reef crystals might be a good choice for a few changes.

Hi all,

I have 2 mixed reef systems, which have been running since November 2025 and March 2025 respectively. Up to this point, I have been using RODI and saltwater purchased from my LFS, which sources natural sea water as live in a coastal city in Florida.

I purchased both RODI and saltwater last week and did my water changes on Saturday. When I tested my water yesterday, I was surprised to see my alkalinity had dropped significantly. Usually, alk would be around 7.8 after a water change which was on the low end of the range but consistent for my tanks,

Biocube 29 + Sump w/ chaeto refugium - I have noticed a green yuma mushroom and my gonis are not puffy/extended

Parameters as of 6/29
Salinity 1.027
pH 8.3
alkalinity 6.7
nitrate 8.3
phosphate 0.44
Ca 430
Mg 1400

Waterbox Clear Mini 6 + HOB - I have noticed some of my mushrooms are not fully open (red)

Parameters as of 6/29
Salinity 1.026
pH 8.4
alkalinity 6.7
nitrate (could not test because the reagent packets were not sealed, unuseable)
phosphate 0.24
Ca 450
Mg 1410

I tested the saltwater that I had purchased and measured salinity as 1.027 and alkalinity as 6.8, which is alarmingly low. I understand that a low alkalinity means my tank is more prone to wild pH swings because of the lowered buffering capacity of the saltwater, but what can I do now if anything? I have baking soda and All-for-Reef on hand, but have never used. Any advice is greatly appreciated. Please let me know if I can provide any further information.

Salinity is tested with Milwaukee Refractometer.
pH, alkalinity, nitrate, and phosphate are tested with Hanna Checkers.
Ca & Mg are tested with Salifert titration test kits.

I do want to make my own saltwater, but am a little lost and overwhelmed at selecting an RODI system and salt mix. Thank you, if you read this far.

As an update, I have received my order of Reef Crystals and I will be mixing my salt water for the first time this week. I will be doing 15% PWC for both tanks and see how we are looking. I think I will give the RO Buddy a shot and upgrade it as necessary, but may continue to research best options before committing. If anyone has any further tips or suggestions as to why the mushroom and gonis may not be puffy and happy looking - please let me know! I want to do right by my critters. Thank you all!