Bob replies...

Hi Ray,

Thanks for your email, and have wanted to address this issue in my column because I've seen something similar asked or commented upon in the past by various authors. And where plenum mechanics were involved, they were, in my opinion, wrong when it came to their comments.

As you're now aware, any appreciable amounts of silicate in the bulk water can lead to a brown diatom alga coating the light exposed sandbed surface, aquarium side panels, and also the surfaces of its corals animals. And this nuisance alga can not only look terrible and consume a lot of effort/time to clean up, it can also block light transmission to the coated corals.

In the wild, this most common element (silica) enters the ocean from land run-off and undersea volcanoes and, as in the aquarium, is used in the structure of the diatom cell wall. And found that anything over 0.2 ppm in closed systems can cause a major diatom bloom. In fact, I've often seen suggestions the aquarium water be tested for silicate, however, in my opinion it's more prudent to test the water used for evaporation makeup and/or water changes. Therefore I've always recommended using processed water for these chores.

Keep in mind the use of silica in diatom formation occurs quite rapidly, therefore aquarium water itself may not show any significant level of silica even though diatoms have or are forming. Yet, testing the aquarium water if severe diatom-like growth exists is useful if for no other reason than to determine whether the growth is a brown diatom or possibly that of a brown dinoflagellate algae, which doesn't require silica for its growth. Phosphate removing media such as aluminum oxide compounds and/or the more efficient newer iron media should be employed.

And since a salt mix may introduce some silica, why add to that with unprocessed water! (In fact, some brands use it to make the salt feel dry and keep it free flowing.) But keep in mind not all RO/DI units, especially household units, do an A-1 processing job. Therefore, those wanting the purest water may want to get some recommendations before they purchase this type equipment. (Visit my website for reviews of said equipment.)

And then there are the aquarists that perform all those tasks and monitor its level, yet still have some diatoms forming on their sandbed surfaces, especially those receiving direct light. As for diatom growth during lighted periods on sandbed surfaces, that is quite normal where there are very active mineralization and nitrification processes occurring. It's caused by silicic acid, a bi-product of the carbonization cycle that is linked with these processes. And of course, said processes will be very active in newly established aquariums, or right after vacuuming or stirring the sand. In fact, those who stir the sand to rid the surface of diatoms so as to have a 'nice' white sand surface, are only encouraging its re-growth and will usually be rewarded within a couple of days with a reoccurrence of the problem!

This now brings me to those who have in the past accomplished very poor testing on plenum beds and either reported spurious results, or simply did not understand its processes while reporting factors for the sake of simply having something to speak of. And I must say, some notable folks have made what they 'thought' were well defined tests, but because they did not fully understand the processes involved, have simply misinterpreted the results they 'thought' they observed. One such plenum test, which used an aquarium feed supply of seawater containing a high level of silicate, showed what was called an unexplainable swing in the silicate levels in the water in the plenum space over a short period of time. Unfortunately, the main focus of the tests was to show what nutrients appeared to be accumulating in the plenum, not how its processes actually controlled and utilized those nutrients, and why said processes occurred. I felt this did the plenum process a disservice, and in fact, its presenter later acknowledged to me that he missed not tracking the most vital aspects of the process.

I mention this particular instance only because it really leads to answering your question as to whether or not some silica sand in the bed will adversely affect the aquarium by generating unwanted diatom alga. And the answer actually came about quite a few years ago (before the above situation) when some individuals badmouthed the plenum process. One of their comments was that there is no way oxygen could possibly find its way through a deep bed of sand and be found in the plenum water space at the bottom of the aquarium. They didn't name names, but just put their misguided statements into print where it spread because it 'seemed' logical. - Didn't everyone know oxygen is used up by the bacteria in the sandbed and that none exists at lower anaerobic levels in the bed! - Well, that may sound correct, and in fact is correct in common sandbeds directly on the bottom of aquaria, however, 'not those in plenum systems!' And 'years' of research had already proven this fact, however, it didn't matter to these individuals, as it seemed they were seeking attention by spouting what seemed like commonsense, yet failing to checkout the facts! In all honestly, how oxygen actually got there was a mystery to Sam Gamble and myself in those early years of our research (92 - 96). So we had to go to a heavy weight in science, a scientist who may be the most knowledgeable person on 'water' in this world, and who consults for the worlds largest companies and many arms of government, including NASA.

Several reasons surfaced, with silica being involved in one of them! What came about where silica was concerned, was that there is an inactive and active side to silica. Its active acids are Monosilicic and Polysilicic acid, which hold 6 - 8 elements of oxygen. Therefore, dissolved silica can deliver/supplement the amount of oxygen that has been recorded in plenum systems (Dr. Craig Jones, pers. com.). And, at the same time, answered the reason why silicate levels varied in the plenum discussion noted above as it was being transformed into dissolved oxygen, which was one of the items the tester failed to test.

And let me take this aspect further because my first plenum system in 1992 used a sandbed consisting of nothing but silica sand. Photos of it can be seen on my website at saltcorner.com in its Photo Gallery. This system always had zero nitrates, and there was 'never' any algae, and in fact, I could not even grow Caulerpa! But my sandbed always had a covering of diatoms, which I did not like, especially when I had visitors coming to see the tank. So, not being much different than other aquarists, I would stir the bed before they arrived and make the bed look more attractive. But the constant diatom growth on the sandbed surface was an annoying situation and I decided to vacuum up as much as possible of this silica sand and replace it with aragonite sand. Well, that finally brought the diatom situation to an end, but little did I know at that time the remaining silica sand was contributing to an enhanced oxygen content in the plenum, and helping to keep my system algae free! So, I certainly wouldn't be concerned about some silica sand remaining in your sandbed! In fact, in the next plenum system that I create, I'm going to 'add' some silica sand to the bed in the lower areas to enhance the oxygen content in the plenum and see if I can recreate that first 'absolutely' algae-free first plenum aquarium!

And in closing, keep in mind that when it comes to cleaning diatom coated surfaces such as the viewing panels, don't just wipe them clean by using a back-and-forth motion with a cleaning pad. This only distributes the diatoms/silicate back into the bulk water where its used again to produce more diatoms on the surface just cleaned. Place the cleaning pad at the bottom of the surface to be cleaned and slowly slide the pad to the water's surface. Then quickly remove the pad and rinse in some clean water. Repeat as necessary. This way, the majority of the diatoms on the viewing panels will be removed from the system water. As for the diatoms on the sand surface, siphon them out, replacing the water removed with freshly prepared seawater.

Hope this helps, and keep me posted.

Bob