Hi sillysidewinder. I'm going to suggest a bit of a backtrack first.
so to cover all bases that might give you a false or correct (but out of ideal range) reading, I'll go through some obvious and not so obvious stuff (appologies if im getting you to suck eggs here).
1. First of all the test kit...Is in date, have you factord the +/- variance of the kit thats usually listed somewhere in the instructions...ie if the variance is +/- 1dkh then your reading may mean 14 or 12. if its 12, then thats not so bad as its just at the very upper end of the normally allowable range (allowable being slightly wider than the standard 8-10 'ideal' range for non ULNS systems).
2. does the kit come with a referance solution that shows you that specific kits true variance (salifert being a case in point)...if not then you need to find/beg/borrow a referance solution to verify the kit, or at least find out how far out it is (most kits are to some degree or other) but its crucial to know if its up or down and by how much.
If that all checks out, then you have a verified DkH level which you can trust and act on accordingly. At the same time (using the kit that youve had verified) mix up a water change and check that to see if the high DkH is being 'caused' by the salt.
At 13dkh i'd be tempted to simply leave be, and let it fall of its own accord which can take several weeks in a low stocked system, rather than rushing in with large water changes (only ever do this if you can see evidence in the stock that they are under stress)...large water changes not only change the chemistry your targeting, but also the rest of the elemental levels which places alot of stress on the whole closed ecosystem (rapid changes to an assumed better level can be just as dangerouse as changes for the worse). the best course is usually several smaller water changes done over a short period so the chemistry shifts/steps slowly back into the target range...
2. One of the hard fast rules of saltwater management is to know for 'deffinate' exactly what your 'true' salinity is, as 'all' elemental levels are relative to the amount of salt dissolved in the water...ie 'salinity' not density....My advice is to ignore density or SG becouse its so temperature dependent, and go strictly by salinity or ppt..as an average you want to be looking for 35-35.5 ppt.
3. the way you measure salinity is crucial. Imo the only two realy reliable ways are with a lab grade hydrometer and long test tube used in accordance with the manufacturers instructions (especialy relating to test water temperature), or more commonly a
refractometer....... and its refractometers that commonly cause the most confusion and most errors in testing...... Just about all refractometers on the market bar 1 i know of are designed for testing brine or saltwater (not 'seawater') which has a different refractive index at 35ppt of around -1.5ppt against brine.
anotherwards, if your testing using a properly calibrated brine refractometer, your 35ppt is in real life 33.5 ppt....this is the reason why so many people complain about various salts being low in Mg etc...Its not actually the salt thats low in alot of cases, its the mix thats weaker than actually assumed...simple answer....mix to 36.5-37ppt if you want a 'true' 35-35.5ppt using a brine refractometer....low and behold, alot of the time your test readings will climb up to where they should be..
Calibration is another common but very big error......When it says 'calibrate at 20c' (or whatever figure is quoted on an ATC (auto temperature compensating) refractometer)...this actually means that the 'body' of the refactometer has to be set to 20c for calibration...NOT the water sample be that RO or distilled...
Once the body has been calibrated at 20c, then the unit will auto compensate for any temperature that the body rises or falls to within its accepted range (usually 5-30C)..ie if you use the instrument in differing environments/rooms/sheds etc etc..
The water sample temperature is pretty much meaningless to be honest becouse once you place just two small drops onto the screen, it is such a small volume that it will equalise to the screen temperature in 5-10 seconds.
I normally recommend calibration as follows for greater accuracy.
take the refractometer and place it in a sealed bag that wont leak...then place it in a jug of water and set the temperature to 20c (or whatever is quoted for calibration) using a thermometer. Leave/keep at that temperature for a minimum of 15-20min to allow the body to equalise its temperature with the surrounding water, then take out, and calibrate with 000 TDs RO water. place a couple of drops on the screen as normal, wait 10-15 seconds and then set to zero.
Then test your tank....If your using a brine refractomer, dont forget to subtract 1.5ppt to get the 'true' salinity.
Once you know your true salinity, you can work out where your levels should be..and diagnose chemistry issues...
The important thing to consider is that when you run a low salinity which is assumed higher, and dose various addatives to boost alkalinity, Mg, or Ca to the correct levels, you are actually running all your 'other' trace element levels at a depleted rate...this is very likely the reason why some people realy struggle with some systems.. allowing for all the variables including incorrect calibration of equipment or poor quality equipment (swing arm hydrometers etc)The salinity is in many cases i have seen far far lower than they believe, so whilst thay add extra Mg/Ca and bicarb to boost the main levels up to where they should be, the rest of the elemental table is severly depleted leading to stress, various unanswerd or unexlainable losses, deprivation, instability, and ultimately a struggling system...
It also explains in alot of cases (actually a huge number) why many people complain about thier salt being low in this and that when its actually not if all the above is taken care of.
on a final note I'll add one last bit of advice...It is very very common for salt in shipping to settle out in the bucket giving rise to widely varying test results for the same salinity from the top of the bucket to the bottom (especially in the case of Mg Ca and DkH)...its absolutely crucial when mixing up a water change that the bucket is rolled or mixed prior to it being used, so that all of the chemicals are evenly distributed throught the bucket....
Equally add salt to water (never water to salt) when making up the batch, and add the salt slowly in small amounts to cold water over several hours to prevent settling and over saturation lower in the water container which will cause precipitation and loss of mg, Ca and DkH. On a 25 gallon change i usually add (spinkle in) the salt required over about 4-5 hours spread across 6 or 7 additions and allways end up with a crystal clear mix with no precipitation at all before bringing up to temperature and circulating for 24hours (remember that thing about refractometers and the water sample temperature..well 35.5ppt is still 35.5ppt regardless of whether the mix is tested at 10c or after its heated to 25c.)
so, my overal advice...dont change anything untill you know for a fact that your readings are 100% accurate...then go from there..
I hope this has been of help.
regards
