Water Chemistry Help

[MaeganWink]

Hello everyone,

 

My students are setting up their saltwater tanks in the classroom. Currently they are at the stage where we've added the saltwater, live rock and sand, and are cycling the tanks. I've had them do some water testing to check the values of the water as time goes on to see what to adjust; the main issue I'm seeing is that the water's dKH is high at 12. The main recommendation I'm seeing is water changes, but is there something else I can do? If the problem is my water source, I'm not sure a water change will help...

 

Other information to note: 

-We used de-ionized water from the Chemistry department as a base with instant ocean for the salt

-Multiple groups of students have gotten the same results, so it's not a testing error in that sense

-I'm using API testkits 

-Outside of bacteria in the live rocks and sand, there is nothing alive in the tanks at the moment. 

 

Any suggestions?

[Origami]

Hi Maegan. What salinity are you mixing up to? And how are you measuring salinity? (Hydrometer? Refractometer? If the latter, is it calibrated with a known-good calibration fluid?) Instant Ocean will mix up to around 10-11 dKH, typically, at 35 ppt salinity. So the 12 dKH is really not that far off (only 18 ppm CaCO3 equivalents, actually). Hydrometers are low cost, but there's a lot that can go wrong with them. Ionic content directly correlates to concentration (salinty), so if you're a little off here, you're alkalinity (and other ion concentrations) will be off accordingly.

 

API is not the most accurate test out there. For a particular lot of kits, though, it's fairly consistent. I used to calibrate API test kits against others that I trusted more (e.g. Hanna & Salifert). By varying the amount of water in the tube, you can effectively calibrate the kit. In general, though, I used to get decent results +/- 1 dKH from API's kit.

 

BTW, 12 dKH is not that bad. It's close to the upper end of what I'm normally comfortable with, but perfectly reasonable for a reef tank. Are you intending to raise calcifying organisms in the tank (e.g. corals, clams, calciferous algae)? If it's a fish-only tank, then feel free to drop the salinity back to 1.023 (about 31 ppt). Otherwise, 34-35 ppt is a good target for corals and such.

[AlanM]

Too much salt mix in the DI water would be my guess too.  That's higher than I ever see Instant Ocean mixing to.

[MaeganWink]

The salinity values do seem to be high when I check the students' notebooks...so I'll have change some of the water to decrease the salinity to see if that helps. Thanks for pointing that out, both of you! We'll see if that resolves the problem. 

[AlanM]

Just add replace some of the existing saltwater with DI.  You can even save it in a bucket for later water changes if you want. 8)