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I'll add that clams are good at getting rid of sand/debris that fall on them. They open and close their shells rapidly to blast away sand and other stuff. So as Rob said -- that is the least of your worries.

Mine do it often. Stupid snails.

Wow. 4-5 ppt??? I did a sanity check because I am a chemist and so lets do a Sunday evening experiment. I quickly made up a 30 ppt and 35 ppt saltwater solution. I checked it with my HI 96822 (which only does a single-point calibration with RODI water). Side-note, if you are looking for the next expensive toy to invest in, I do recommend this fancy refractometer. Talk about zero-guess work! http://www.hannainst.com/USA/prods2.cfm?id=039001&ProdCode=HI%2096822 My portable refractometer reads 0 with RODI, 30 ppt and 35 ppt. I did it three times. Each time I spun my calibration screw a half turn and the performed a single-point calibration at either 0, 30 or 35 ppt. My refractometer is model RHS-10ATC, if you want to search for it. Here is a protocol from South Carolina: http://score.dnr.sc.gov/ktmlpro10/files/uploads/elearning/refractometer.pdf I get the same readings (+- 1 ppt error, which is the best we are going to get for under $50) no matter what I concentration I use for my single-point calibration.

And I agree with Randy: "As with many devices, however, you sometimes get what you pay for, and sometimes less. Very inexpensive refractometers can be prone to errors and may need to be checked in a solution matching seawater, not just pure freshwater." So spend $35 and get a refractometer that is high quality.

Randy does an excellent job of talking about the linear nature of these devices. The take home message is that you need to know how well your refractometer was calibrated. Once you convince yourself that your refractometer is correctly factory calibrated, then there is no reason to use 35 ppt standard. It is not wrong to do so, just unnecessary. If you wanted to save $$ and bought a bad refractometer, then yes, you MUST use a 35ppt solution.

Why yes. It does help. If you read down to the section "Tips on Calibrating a Refractometer" you will see my point. Most refractometers will not have those errors. Once you prove to yourself that you purchased a decent device, RODI water is fine for a blank.

<sigh> waste of time.

That's my point. Your tank would be 37 ppt (uh oh...) whether you used your 35 ppt solution or RODI water to single-point calibrate. But RODI water is easy, costs nothing, and there is little doubt that zero is zero.

Try it!

I just want to be clear. There is nothing wrong using a standard solution. It is just more work, time and expense. My point is that it is unnecessary. In many optical readings, a blank works just fine as means of rough calibration. For our needs, a blank is sufficient.

Yes. But we are not two point calibrating anything. We are one point calibrating a linear scale. Do you calibrate your bathroom scale with a standard weight? Or just make sure it says zero when you are not standing on it Whatever. When I put rodi water on my refractometer it reads zero. When I put 35ppt on my refractometer it reads 35 ppt.

Do a google search or YouTube search on refractometers and how to use them. http://maple.dnr.cornell.edu/kids/refractometer.htm http://www.homebrewstuff.com/refractometer-how-to Among others

Also, we cannot change the scale on the refractometer. If the factory screwed up a ruler, then can recalibrate and make a new scale with a sharpie. You can re-etch the thermometer. But you cannot change the scale of a refractometer. Thus you cannot 2 point calibrate the device. You can only zero, or a one point calibration.

No. You are wrong. Well, look if you are ok with using an inaccurate ruler, then I got nothing add. I like to buy rulers that are correct without the need to spend time calibrating. I zero the ruler at the beginning of what I measure and read out how long the distance is like almost everyone else. I don't go to NIST and calibrate my ruler against a standard foot long rod, re etch my scale, and then make my measurement. Think of everything that use daily that we use zero as a reference. Scales. Thermometers. Levels. Rulers. Speedometers. Odometers. There is nothing fundamentally different about a refractometer that warrants treating it otherwise. In fact, if you treat it carefully and don't knock it or drop it, you shouldn't need to zero often. We zero because the lens can move over time. That's all. The ph scale is not a linear scale. It is logarithmic. If you think a refractometer is to calibrated like a pH probe, then we have identified how your logic is faulty.

I am assuming the prism and scale are accurate. If you buy a refractometer from a company that does not have a reputation for producing an accurate device then you are wasting your money and your time. If you buy a 10 cent ruler, but 12 inches does equal 12 inches, then what is the point? Most refractometers that you will find at reputable hobby stores are accurate. You can zero at zero with rodi and save time and worry. Check the reviews. If you go on eBay and buy a ten dollar device from Slick Willies' discount things from a stolen truck, then all bets are off. All measuring devices: ruler, level, thermometer, etc. are calibrated by factory workers. The better trained, the higher the salary and thus higher the price. You get what you pay for.

Yes. The easiest way to be precise with respect to temp is this: Add a drop or two of water on the prism. Remove air and close cover slip. Wait 30 seconds. This will equalize the temperature between the prism and the water. If you want to see the effect of temp in the reading, then put a drop of cold tank water on the prism and look through the refractometer as the water warms up. It is not dramatic, but the line does move.

I disagree. The precision of the refractometer does not change given same temp and consistent measuring practices. In other words, the error deviation does not change depending on how one moves the scale. The error is the same and mostly due to user habits. The hobby devices do not have high magnification to the scale and reduces the accuracy. Zeroing at 35ppt does not increase accuracy significantly. High end refractometers use high end microscopy optics to zoom into the scale. They also have better prisms and optics as well. That leads to better accuracy. Well trained analysts lead to great precision. Let's be clear. When you single point calibrate the device, all you are doing is moving the prism to align the printed scale with the refraction line. Since you are not changing the prism's shape (and thus refraction physics) nor the rulered scale, you do not affect the precision of the device. The situation you described with the thermometer is indicative of a work around with a faulty thermometer and not proper practice. Not standard procedure. If you etch 100c in boiling water and put it in ice water and mark 0c, then when you put it in boiling water it should read 100c. If it does, not then it is non functional. Usually this happens when a bubble is introduced into the liquid.

This comes up from time to time. I'm a scientist, and I use RO water to zero my refractometer. It is most likely that your 35 ppt is not actually 35 ppt. If some water evaporated, then the concentration would be higher. I'm a scientist, so forgive me for being anal-retentive. The refractometer's optics are calibrated at the factory and hobby-grade refractometers cannot be multi-point calibrated by the user. At the factory, they use standard solutions to allow them to place the scale sticker in the right place on a lens. This principle is just like an analog thermometer. If you watch how they make thermometers, they etch the scale on last. http://www.sciencechannel.com/tv-shows/how-its-made/videos/how-its-made-thermometers.htm The end users cannot actually calibrate a thermometer. If you boil water, and stick in a thermometer and it reads 50 degrees (Celsius or Fahrenheit) then there is nothing you can do but throw it away. What we do is a "single-point calibration" or zero the device. It doesn't matter if you zero with RO water or with calibration solution. It is easier to zero with RO water because it is more constant. RO water will be always zero. There is a good argument as to why to use 35 ppt calibration fluid for more accuracy in professional settings. But our hobby grade devices do not have the resolution to take advantage of high accuracy. For example, we need to know if the water is 35 ppt. We don't need to know if it is 34.8 or 35.2, nor can our devices give us that information. So, zero with RO water and worry about one less thing in this hobby.

Eggs can be quite hardy and survive some extreme conditions.

I'll add some more science. If the scale is clear, then the device is in focus. If you want a clear sharp refraction line, there are two thing to consider: 1) The film of saltwater must be thin. I load a few drops on the glass surface and press down on the lid, squeezing out the excess. Check that there are no air bubbles. While reading I press down on the lid (from the edges) and the line sharpens. 2) Light source. Point directly at the brightest light you can find. The brighter the light, the sharper the line will appear.

This.

Cool piece!

If the color is indeed in the glass, and not painted on, then you will be fine. Glass is inert and so the migration rates of heavy elements out of the glass and into water is so small that it is not worthy of concern. Many scientific studies have been done to show that metals stay put in glass. (e.g. will the cobalt blue bottle that holds my liquor eventually poison me? Nope.) Paint on glass is a whole other issue. But even in that case, a quick treatment with clear sealant would solve the problem. With some spray sealant you can put anything in the tank.

I don't know. Maybe high nitrites is a correlation, and not a causation, of coral decline. Personally, I don't understand how potassium phosphate is helping the corals thrive. Potassium is an essential element, but is that enough to explain tremendous coral coloration and growth? Are we hurting our coral by aggressively removing phosphate? And where the heck is the nitrate coming from? Excess feeding causes an increase in *both* nitrate and phosphate (often more phosphate). Urine is one plausible and gross idea. I can understand starting with high nitrates. But after numerous large water changes and phosphate addition -- it should not be climbing out of control while the phosphate levels remain low. I mentioned this to three oceanographers and they had no idea what could be wrong. Could the phosphate be absorbed into the rock? I think that requires a higher pH, low pH release phosphate from the rock.

I don't believe such a skimmer exists. The collection cup always interferes with the lid. You'd have to remove the collection cup each time you open the lid. I use the aquamaxx hob-1 Here are some of your options: