Collection of Representative Water Samples

[jgranata13]

I have a mind to do a series of tests on how diffusion works in a typical reef tank, but I thought I'd check on here if anyone else has already done something similar.

My specific worry is this: Every hour my ATO adds freshwater to the tank. When I'm collecting a water sample to test my parameters, how do I know that there's not an abnormally high proportion of freshwater in the sample? I do have reason to believe that this question is of practical - rather than just theoretical - importance. More on that at the end of this post. Our systems are hardly examples of perfect mixing - two isolated boxes connected by comparatively small pipes, a circular current (not at all efficient for mixing), and both the tank and sump each containing many barriers to water flow. Diffusion in such a system is a VERY complicated process, so I think the only way that you could ever be sure that a sample is representative would be to determine the amount of time that it takes for the water column to become uniform again after a point disruption in chemical makeup (e.g. the addition of topoff water, a dose of some additive, etc.), and collect a sample only after knowing that no disruptions have occurred for that amount of time.

Has anyone ever determined that critical time for their tank?

**********
This idea was precipitated the fact that since I started testing for them, my mg/alk/ca readings have been wildly inconsistent over consecutive days - fluctuating up and down even though I don't add anything on a daily basis. I'm pulling my hair out trying to figure out why! I'm a math/science person and very good at experimental design and controlling extraneous variables, so I'm really confident that it's not user error. I've tested each test kit (they're all Red Sea Pro) against itself for consistency, and they all passed with flying colours; I set a testing protocol more specific than the ones given in the instructions, and adhere to it every time; after using a testing vial once, I clean it first with 10% vinegar and then with a saturated baking soda solution (which is very basic), and then rinse all equipment with RO/DI water. There's literally nothing else it can be except an issue of diffusion. So, let's discuss our experiences!

 

[Vinny Kreyling]

Test sample should be as far away from the ATO as possible.
Not much else we can do. Unless something goes out of whack my tests on a weekly basis are very consistent.
If I had to experiment it would start out @ 4 hrs & go from there, more time or less depending on results.

 

[Rover88]

Too many variables to really do definitively well...


Best option would be to set up something akin to a test tank, entirely filled with RODI water and nothing else, then add in a large amount of dye of some color or other that is soluble to the water and watch what happens. See how long it takes for the water to take on a uniform color, but also get to watch how/where the mixing takes the longest, perhaps with a time lapse or something of the sort.


Our test kits are inaccurate enough that even if in a normal working reef tank we took samples from the upper corners, lower corners, middle of the tank, each of the skimmer chambers, the chamber the ATO feeds too, etc... Well, the kits variance is enough that it may well rule over any variances you see in the actual water parameters of the tank itself.


That being said, O2 saturation is something else to consider. O2 saturates most heavily towards the top of the tank normally where the air is, and there will be a sliding scale of pH from the top to the bottom of the tank, and these pH swings will affect your calk/mag/alkalinity unless I'm mistaken, so even more variation comes from that.

Temperature affects O2 saturation, so if you are like me with your heaters in the sump (where the skimmer is, which also aerates the water) then you'll be blasting warmer well aerated water into the slightly cooler top tank with less aerated water, which will make waves of inconsistencies...


This is why our tests are really just benchmarks to see where its at, and you can't look at any one point so much as the 'trends'.

 

[Dan_P]

Not sure what you mean by "œagainst itself for consistency". Do you mean you sample the tank and test it three times, or did you sample the tank three times and tested each sampling once?

You did not mention the size of the day to day variation that is bothering you. How much larger is it than the same day, test to test variation, if you ran triplicates?

You also didn't mention how old your system is and whether the variation is relatively new or an old issue.

Thinking over sources of error a bit more might be informative, otherwise, use a nontoxic dye to determine how long it takes to mix in the aquarium. You will need to be able to measure the color. Dosing a slug of calcium or magnesium chloride would be another way to monitor when your system reaches equilibrium if you trust you measurement kits.