diatoms

Question
Hi,
I recently purchased a 75 gallon tank that was up and running for 2 years complete with live sand, live rock, soft corals, a protein skimmer, sump tank in which I keep carbon, filter floss, and chemi pure, dual actinic lights and white halide lights, that are on 9 hrs daily.  Occupants include a yellow tang that was included with the tank, royal gramma, clown fish and flame hawkfish, that I added, as well as several hermit crabs, snails and a brittle starfish.  My problem is a week after setting up the tank, I noticed a rust colored film covering the sand, rocks and walls of the tank. I do a 20% water change weekly. I have well water, but I add the chlorine remover drops. The water values are all normal except the nitrates are a little high - 10. I've been told the rust film are diatoms caused by silicate in the water and I bought purigen which is supposed to remove all harmful chemicals in the tank but so far, no luck. Any help would be greatly appreciated.

Answer
Hi Cathy,
Without pictures it will be hard to be specific. Are they velvety looking, hairy looking or just slimy looking? They could be diatoms, or dinoflagellate, but I think more likely a cyanobacteria. Cyano can be rusty, red, burgundy, purple, green, brown or black in color. It appears quickly in a nutrient rich environment and can spread fast. Elevated nitrates, phosphates and a new system are a good place for it to start. A salifert test kit from your local fish store can give you the ability to test for silicates, calcium, phosphates and other important parameters in your water quality, I'd pick one up, it'll be worth it in the long run. The best way to get either under control is to manually remove as much as possible. Vac the substrate, just deep enough to pull the nuisance from the substrate, clean/scrape the glass and you can use a power head to blow it off the rocks or actually remove the rock, scrub it off with a clean brush, rinse  the rock and put it back. Wait a few days then clean your sumps media. This way you dont disrupt the biological filter too much. A weekly water change shouldnt be necessary. You should be able to do a 25% change evry 3 weeks and be fine. With the problem at hand a larger change will be a great help, 40 to 50%. Test your water source that your using as well. Make sure that it's parameters are in order. Sometimes our drinking water is pretty dirty by our aquatic pets standards and creates quality issues. Ideally you should use ro/di water for mixing and top off. Even ro/di water needs to be tested for ammonia though. Get yourself the salifert test kit, then I would do the cleaning, a 40 to 50% water change and test. Reduce your lighting to no more than 8 hours a day and cut back on the amount of food in each feeding. You'll be able to get it under control quickly. Let me know how it goes or if you have any more questions.