Bob replies...

Hi David,

The rusty film could be either a diatom alga or cyanobacteria. Lets first consider diatoms, which use silica (silicate/silicic acid) in the structure of their cell walls, and depending upon the amount in solution their growth can appear dot or mat/film-like. In especially new aquariums, the substrate surface bacteria are quite active in colonizing their areas and in doing so produce small amounts of silicic acid, a compound that promotes the growth of brown diatom alga. If there are not other inputs of this compound, e.g., from unprocessed water, the diatoms will usually dissipate within a few weeks. But if you constantly rake the bottom in the quest to maintain a white lagoon-looking bottom area, you're upsetting the bacteria in those areas and forcing them to double their efforts so to speak to recolonize those disturbed areas. And in doing so, they are again producing silicic acid as a byproduct of their processes and presto, diatom algae keeps reappearing.

Nevertheless, diatom growth should not be allowed to remain if it starts to form mat-like growths, as it can interfere with diffusion. When and if mats form, vacuum or siphon out just the upper grains as necessary. Do not stir or rake the surface. Otherwise, bear with it for a few weeks and see if it diminishes. If not, look towards the quality of the water being used for evaporation make-up or water changes, as it may well be the source of the problem. If so, then RO/DI water is needed to help resolve this aspect. Furthermore, keep in mind that anything composed of or containing silica, e.g., silica sand, Diatomaceous Earth, or possibly some decorative rock, will has a slight solvency at the pH normally found in marine aquariums. Again, if it's an on-going diatom problem the incoming water supply is probably the first item to checkout.

If it's cyanobacteria, very small amounts should be considered an attention getter, as water quality issues should then be addressed, e.g., nitrate/phosphate levels. As for the root cause, overfeeding is probably the number one cause. Add to that poor water flow and lack of removing excessive detritus, and the conditions are there for some unsightly cyano growths. Better overall aquarium husbandry, e.g., improved water movement; controlling what and how much is fed, using a quality protein skimmer; and, reducing nitrate/phosphate levels, all of which are helpful in reducing or eliminating this pest. As for phosphate, it should be >0.015 ppm and in the UK, ROWAphos (also good from removing silica) is probably the number one phosphate remover, and would suggest using it in either a phosphate reactor or canister filter, as its more cost-effective this way than just placing it in sack and placing that somewhere in the system.

And please, if this turns out to be cyano, which I don't think it is, do not allow yourself to be talked into using an antibiotic, e.g., Erythromycin sulfate or Tetracycline, to kill this bacteria. Even though these chemicals may eliminate what some call a pest algae the first time around, further problem blooms may become more resistant to the drugs. It is really much wiser to get to the root of the problem than to put antibiotics into the aquarium in these type situations. Bear in mind antibiotics also negatively affect system biological filtration! If necessary, I would recommend Boyd's Chemi-Clean if cyano really becomes a problem, then proceed to correct the root problem areas as mentioned above, then after that, add two teaspoons of brown sugar monthly to maintain an aquarium free of cyano.

As for a sunrise and sunset photoperiod, would recommend the Actinics coming on about 10 AM, the white lamps a half hour later, then the main system metal halide about noon. Depending upon your viewing period, the metal halide could go off at 9 or 10 PM. A half hour later the white lamps should go out with the Actinic following thirty minutes later.

I hope this helps,

Bob