Wacky water and colony reduction


New member
I finally got some test kits and was surprised by my findings.

Ph = 8.0
Salinity = 1.021
Ca = 320
Kh = 13.1
Alk = 4.69
Iodide = 0
Strontium = 0
Mg = 900

There are mainly SPS in this 20g tank under T5 lighting. I was noticing a reduction in sizes of larger colonies although the individual polyps look good. I am guessing that my water is way out of wack. I was dosing with a "do-all" for nano reefs but with readings like this I have stopped that and gone to dosing for individual readings.

I don't know how long it will take to get readings to normal. I hope my coral hangs in there.


New member
what kind of salt are you using?

you should start by getting your salinity up to about 1.025

strontium, iodide, and kh don't really need to be tested, focus on alk, mag, and cal which are all dangerously low

Randy Holmes-Farley

Reef Chemist
Premium Member
What additive were you using? The alk may drop as you stop using it.

I'd raise salinity to about 1.0264, which will bring up calcium (to 400 ppm) and magnesium (to 1130 ppm), although magnesium might need an additional boost.

Don't bother with iodine supplements unless you keep gorgonia.


New member
I was getting saltwater already mixed from the LFS so I don't have a clue what they use.

So SPS likes salinity at 1.025? 20g is such a small tank that the salinity will swing due to evaporation from 1.021 to 1.025. I can certainly let it stay saltier though. Is there an upper limit not to exceed?

Randy Holmes-Farley

Reef Chemist
Premium Member
I discuss salinity here:

The “How To” Guide to Reef Aquarium Chemistry for Beginners, Part 1: The Salt Water Itself

from it:


What Salinity to Use?


For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. Salinity, however, does vary substantially from place to place.

As far as I know, little real evidence suggests that keeping a coral reef aquarium at anything other than natural salinity levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. Substantial misunderstandings also arise among aquarists as to how specific gravity really relates to salinity, especially considering temperature effects.

The salinity on natural reefs has been discussed in previous articles. My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).

Fortunately, coral reef aquaria seem rather forgiving with respect to salinity. The range of salinities encountered in what most would proclaim as successful reef aquaria is actually quite large. Don’t agonize over small deviations from natural seawater. You will not notice any benefit changing from 36 or 34 ppt to 35 ppt (specific gravity = 1.0256 to 1.0271). Many fine reef aquaria appear to run at salinity levels as low as 31 ppt (specific gravity = 1.023), but bear in mind that the values that aquarists report (as well as your own measurements) are fairly likely to be inaccurate, so pushing the low or high end of the range may not be prudent.

Bear in mind that if aquarists target salinity values different than 35 ppt, the amounts of calcium, magnesium, alkalinity, etc., will all likely deviate from natural levels as well. For example, making artificial seawater to a low salinity will normally result in low values for these parameters and may require adjustments.


New member
New readings for 20g:

Salinity 1.0255
Mg 1140
Ph 7.8
Kh 12.5
Alk 4.46
Ca 400
Temp 78 with lights on

These certainly seem better, but I am wondering if I should be adding anything at this point.

Randy, I did read more of your articles. I don't know if I read them all though.


Premium Member
I'd dose the magnesium up to 1285 ppm, and let alkalinity fall to 4.0 meq/L or so, but the tank should be okay as is. The water parameters article should help.


New member
Today Mg = 1200. I had to go get more from LFS.

Here are the sad pictures.