Sam Mancini writes...

Hi Bob:

Haven't talked with you in a while, so I hope you're doing well. I'm hoping you can help me with a frustrating problem that I'm having with a 180 gallon marine aquarium that I set up during the first week of January. It is a fish only system, with approximately 183 pounds of live rock and a 4" sand substrate. I decided not to use a plenum on this system, though I presently have successful marine and reef tanks using both the plenum and deep sand bed methods. I've learned that both systems work just fine if they are properly set up and cared for. Beneath this tank is a 50 gallon sump that houses a protein skimmer, a pair of heaters, and a large return water pump.

The tank contents come from a 46 gallon bowfront aquarium that I subsequently sold to a co-worker. Fifty pounds of live rock, 25 gallons of saltwater, and the three resident fish from that tank were all moved to this new system when it was being set up. The additional live rock was purchased cured from local dealers and was of the highest quality. The remaining saltwater (about 120 gallons) was mixed fresh using Tropic Marin salt. The three fish were well established in their prior system, and include a pygmy angelfish (about 1-1/2"), a Scott's fairy wrasse (about 4"), and a purple tang (about 4").

Here's the problem. I realized that a diatom outbreak was going to occur, and it did show up roughly two weeks after the tank was up and running. However, it is showing no signs of letting up and the tank is now six weeks old. In fact, it is getting darker and taking a better hold on the sand substrate with each passing day. Experience tells me to "wait it out" and eventually it will regress. However, I would like to help the process along if at all possible. So far, these growths are well established on the sand bed -- especially in those areas of lesser water flow. Only minor traces are seen on the live rock and on the glass. Lighter areas are a golden brown color, and darker areas range from brown to dark brown.

To battle this problem, I have taken the following measures to this point:

1. Added a variety of snails including 100 nassarius snails to aerate the sand bed, 20 cerith snails, 15 astrea snails, and 5 large trochus snails to graze on the diatom growth -- and other potential growths when they appear;

2. Added two white sand-sifting starfish to further aerate the sand bed and turn over the surface;

3. Installed a AquaClear 500 Power Filter directly to the tank with both carbon and PhosBan running through it;

4. Added a third powerhead to the back wall of the tank (joining the two powerheads already in place, and bringing the flow from the powerheads up to 890 GPH). The power heads are placed in the upper third of the tank; if placed any lower they blow the sand bed into suspension;

5. Cut back on the 50/50 power compact lights (from 10 hours per day to 6). The standard fluorescent actinic blue lights still run for 12 hours each day;

6. Cranked-up the already efficient protein skimmer to produce a steady, stable foam.

Adding and installing these items gets expensive, and they've not yielded the results I have been hoping for. The nassarius snails burrow deep into the sand bed, and do not turn over the sand's upper layer to disrupt the diatom growth. The other snails stick to the rock or glass, and rarely move across the sand to feed. The white sand-sifting starfish pop-up every now and then, but not enough to stir the sand surface on a regular basis. The flow from the third powerhead is good, but in a tank this size you would need 10 powerheads to truly eliminate any stagnant areas. And, too many powerheads look bad and can get expensive. The lighting is a minor player, so the cut back from 10 to 6 hours produces little results.

Silicate does not appear to be the problem. The sand I used is sugar-sized and "silica free" according to the packaging. To verify this I tested the water with a Salifert Silica Test Kit, and all of the test results were zero. Other test parameters appear fine (8.2 pH, 9 dKH, etc.). The total flow in the tank is now somewhere around 2,100 to 2,200 GPH, so in my estimation the flow is more than adequate. All of the top-off water is from my SpectraPure RO/DI unit, and when checked this weekend with my TDS meter no detectable impurities register (only a zero '0' shows on the monitor).

I'm at a loss to explain why the diatoms continue despite all of these measures. Am I being too impatient, or should I step up the fight against them? Should I try to siphon the sand bed, though this means a large amount of sand will get siphoned out of the tank? I'm just about 'crittered-out,' but are there any other sand stirrers that might help here? Maybe something that stirs the sand's surface on a regular basis? Manually stirring the upper level of the sand bed makes it look good for a day or two, but the brown growth comes back without exception. Is there anything you can recommend? I'm open to any and all suggestions you might have!

As always, I appreciate your input and advice.

Sincerely,

Sam Mancini