Diatoms are sometimes referred to as microalgae, brown algae, or golden algae and can be very unsightly to look at in a reef tank. We are currently experiencing a “bloom” of diatoms and researching the possible causes and solutions.
From my reading, these are the possible causes of a diatom bloom:
Having a tank with silica present can cause diatom problems that remain present until all of the silica is used up. A common silica source is certain types of substrate, such as playground sand or other substrate that does not come from the ocean, tap water, or even low quality salt mixes. I do not believe that silica is causing our bloom, as the substrate we have is 100% aragonite from the ocean, we use a quality salt mix, and use RO/DI water.
We added a new piece of live rock to our tank recently, to create a higher place for some of our corals that need more light. As some of the organisms on this new rock die off, it adds nutrients to the tank that create an environment that is perfect for algae to grow. We believe this may be our main culprit. The only other time we have had diatoms were when we first set up our tank and it finished cycling.
Another reason may be due to overfeeding, causing excess fish food to decompose and add nutrients these diatoms love. However, we are quite cautious about feeding, as we were warned that overfeeding is a bad habit of new aquarists that can cause all kinds of problems, including algae growth.
One other possible culprit, which may take some further investigation to prove, is phosphates. Our phosphates are consistently low or not present when we perform water testing, and we are using a product called PhosGuard by Seechem which is placed in a mesh bag in our sump. This products removes specifically phosphate as well as silicate if they are present in the tank.
So where does this excess phosphate come from? Mostly fish food or tap water. Since we do well with not over-feeding and we use RO/DI water, what else could cause excessive phosphates, if this is in fact, causing the diatom problem? We performed testing on the RO/DI we keep in our food-grade container for daily water top off, as well as our tap water, and both tested positive for small amounts of phosphates. Is it possible that our RO/DI system is not removing all of the phosphates, or even possibly that the container we are using is leaching phosphates? The container we use is PP 5 food grade container, and from my reading should pose a low risk of leaching as it is often the type of plastic used in ketchup bottles and yogurt containers. So maybe our RO/DI unit is not doing it’s job. The next step is to test the RO/DI water directly from the unit. We can test for phosphates at home, but have decided that on the next trip to Preuss Pets we will take water samples from the display tank, the RO/DI holding container, and directly from the RO/DI filter and compare our home tests with theirs.
Until then, we have found that maintaining regular water changes as well as vacuuming the substrate and rocks have been helpful. We also cleaned the in-flow and out-flow sections of our sump of any detritus on our last water change just in case. This is certainly not an over-night fix and we will continue to monitor and troubleshoot this issue.