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Brown Dusting in Reef Tank

A brown dusting in a reef tank is a result of too many nutrients.

By J. Charles Delbeek, M.Sc.

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Aquarium Fish International Magazine
Q. I am new to the saltwater side of the aquarium hobby. I have had my aquarium up and running for four months now. It is a 14-gallon aquarium, and I have a trumpet coral (Caulastrea), a Ricordea mushroom, green star polyp (Briareum) and colony polyp (Zoanthus). I have live rock, one coral banded shrimp, two sand snails (Nassarius), two margarita snails (Margarites pupillus) and three small saltwater fish.

Click image to enlarge
Trumpet coral can be fed microscopic foods or even krill
This trumpet coral (Caulastrea sp.) can benefit from microscopic foods. However, as a large-polyped coral, it can be fed krill or mysids, too.
Local water authorities can test your silicates ppm
When it comes to silicates, anything more than a few parts per million can be a problem. Your local water authority should be able to give you a free water analysis.

I feed my aquarium once a day in very small amounts, but sometimes I will give the fish some krill in the late afternoon. I use a product called Reef Bugs to feed my coral, being careful with the amount I feed them. I recently had a very bad problem with diatoms and cyanobacteria. I used a slime remover, and my aquarium was great after treatment. I use a phosphate-removing pad, but this seems to coat everything with a brown dusting. Is this ferric oxide or just diatoms?

I do weekly water changes with prepared saltwater with reverse-osmosis (RO) water that I buy from a local aquarium store. My aquarium is just this ugly brown with algae on the live rock. The only time it was a great-looking saltwater setup (a pretty green) was right after the slime treatment and also when I first set it up. If the filter in the RO is not maintained and changed, could the RO water contain silica?

My phosphate tests zero and nitrate is less than 5 parts per million (ppm). Specific gravity is 1.025 and pH is 8.2. I use a UV sterilizer and run it 24/7; my protein skimmer and compact fluorescent bulbs run for 11 hours a day. Right now, I have a poly pad, and the skimmer as my form of filtration. I haven’t been using any reef supplements because I want to first remedy what is going on in my aquarium.
Janet Wimbert
Bardonia, New York

A. There are a few things that I would comment on with regards to your system. First, doing weekly water changes may be introducing excess nutrients, primarily silicate, to your water. You might want to cut back to once a month to see if this helps; this will give time for your system to stabilize between changes and will allow any excess nutrients to deplete.

I have never used Reef Bugs, but I have to wonder what you feel you are feeding with this microscopic feed. No animal in your aquarium really needs to be fed this. The one exception might be the trumpet coral, but its large polyps can easily be fed with pieces of krill or Mysis when they are open. As I said, I am not familiar with the product, other than what I have read on the manufacturer’s website and in forums. It may contain something that is fueling your marine algae growth, so if you stop adding it for a month or two, you should be able to see if it is having any affect on your system or animals.

Yes, a faulty RO membrane may be to blame, but you may also live in an area that has high silicate values in the tap water. You should contact your local water authority and request a free analysis of the water your store receives. Anything more than a few ppm would be a problem. The RO membrane can remove some silicate, but if the water is high in silicate, they may need to add secondary deionization cartridges to remove the remaining silicate. Silicate levels can also vary during the course of the year, depending on if your water source is affected by rainfall or agricultural runoff.

Although you mention lighting, you don’t state how many watts of lighting you have. Eleven hours a day of light is a bit much in my opinion, depending on how bright it is. You might consider cutting this back to eight hours (or even six) to see how this affects the aquarium. Just run the lighting during the hours you are most likely be at home viewing the aquarium, for example, from 2 p.m. to 10 p.m.

Although you mention some water chemistry parameters, you do not mention the two most important ones for reef aquariums: calcium and alkalinity. The fact that you say the aquarium used to be a “pretty green” color makes me wonder what these values are. Usually, sufficient calcium and alkalinity, combined with proper pH and magnesium levels, will result in the proliferation of pink and purple coralline algae on the aquarium walls, plumbing and rocks. You make no mention of how you maintain calcium and alkalinity, so I have to wonder if you do this. If not, try some of the two-part additives out there. Your calcium should be between 380 and 420 ppm, and alkalinity should be between 2.5 and 3.0 milliequivalents per liter.

Finally, the red “dust” you mention could be coming from the pad. The easiest solution is to place a second pad of filter floss after it to trap any residual particles that might be coming off the ferric oxide pad.

Although your water tests zero for phosphate, this really isn’t the case. It is impossible to have a reading of zero — all the test kit is telling you is that the phosphate level is lower than the kit is able to detect. So if you know the lowest level your kit can measure down to, you will know that your phosphate level is less than that value but that is all. Phosphate should be 0.02 ppm or less. Unfortunately, few test kits can even measure this low. I would try the suggestions I made concerning water changes, aquarium lighting and maintaining calcium and alkalinity before worrying much about phosphate.

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