Rising Salinity and High Magnesium - Beginner questions

[Punchanello]

Hi All,

My tank has been running for 4 months now. I keep mainly LPS at this point. I use RODI water and Aquaforest Reef Salt. I regularly calibrate my refractometer and do water changes. Generally I just siphon out detritus in my sump and display and replace the same amount. This works out to about 10% of the system volume a month.

My parameters are -

Nitrate and Phosphate - 0 (salifert)
Calcium - 425 (aquaforest)
Alkalinity - 7.7 (aquaforest)
Magnesium - 1500 (aquaforest)
Salinity - 37ppt

So here is my story -

I measured my big three for the first time so I could try to work out how much I need to be dosing. I've relied until now on small water changes with Aquaforest Reef Salt. According to Aquaforest at 35ppt (which is what I have been shooting for) my calcium is a little low and magnesium is very high.

It might be just user error but I'm pretty meticulous with measuring water for water changes and re-calibrate my refractometer regularly. Is there something I might not be accounting for that is causing my salinity to creep? Also, is the high magnesium explained by the increased salinity (the salt mix contains magnesium)?

Once my salinity has come down I want to use Aquaforest's balling method. How do I go about lining up my numbers (reducing magnesium and increasing alkalinity) so I that I can dose equal amounts of each part?

 

[vegamedic]

What type of water are you topping off (replacing evaporated water) with? I assume RO/DI as well?

 

[Punchanello]

What type of water are you topping off (replacing evaporated water) with? I assume RO/DI as well?

Yes RODI

 

[just4plaay]

Are you topping off with plain RODI water or mixed saltwater?

 

[vegamedic]

Yes RODI

pre-edit-edit: you might want to get rid of that pink/red table you have posted. I assumed it was your follow-up test results and not a params list from your salt manufacturer and others might too

In the absence of significant dosing, on a small time frame there's really only one way I can think of that your salinity would rise, which is adding more saltwater back into the tank than you're pulling out. That's the nuts and bolts of the salinity issue, how you fix that is going to depend entirely on how you top-up RODI and your water change procedures.

Once my salinity has come down I want to use Aquaforest's balling method. How do I go about lining up my numbers (reducing magnesium and increasing alkalinity) so I that I can dose equal amounts of each part?

you can dose any number of alkalinity supplements to raise it, check in the reef chemistry forum for recipes. as for mag, don't worry about it.

 

[Randy Holmes-Farley Verified Community Expert]

What are you using now as a supplement? Most alk and calcium supplements slowly raise salinity.

There's no way to reduce magnesium except by lowering salinity (1500 ---> 1419 if you drop 37 ppt to 35 ppt) or by water changes with a lower magnesium mix (not efficient and you mix is likely how it got where it is now).

You are using 35 ppt water for changes, and are concerned that the tank is 37 ppt?

It might be a measurement error if the temperature is not being properly accounted for by the refractometer, but the fix is trivial: remove some tank water and add back RO/DI (or let the ATO do it). To drop 37 to 35 ppt, you need to replace 5.7% of the water volume (~5 gallons in a 100 gallon tank).