Purple slime algae

[Derek L]

Hi all, 

Looking for help. 65L marine tank established for 18months. Only traces of nitrate and phosphate with 20% water changes weekly using fresh warmed sea water (tested and fine). Past few months bright purple slime has taken over. Cleaned and largely removed weekly but reappears within hours. Corals OK but not really thriving. No fish in the tank only a blood red shrimp, cleaner shrimp and boxer shrimp with an emerald crab an 3 hermits. Sand cleaned and rocks scrubed but problem seems to be getting worse - any ideas

[seabass]

Welcome to Nano-Reef.com.

 

The purple-red slime is cyanobacteria.  It's likely consuming a bunch of that phosphate and nitrate in the water.  But having low nutrient levels isn't typically healthy for coral (which as photosynthetic organisms, need these nutrients for energy).  Phosphate should normally be between 0.03 and 0.10 ppm; and nitrate between 3 and 10 ppm.

 

Cyano is often caused by excess organics, which tend to settle in low flow areas of your tank.  So people often consider cyano a flow problem.  However, it's typically an organics problem (that might be improved by adding more flow).  A bacterial imbalance can be another cause but it's still typically related to organics.

 

Without knowing much more about your system, I might recommend more flow and more maintenance.  The sand bed might have become saturated with organics which is fueling the cyano bloom.  Use a turkey baster (or small pump) to blow detritus off of the bare rock, then siphon the detritus of of the top of the sand bed.  Do this every few days until you notice improvement.

 

As a more extreme measure, you might decide to clean the substrate by:

• Removing and saving as much water as you can from your tank (before you disturb the sand bed).
• Placing your livestock, coral, and rocks into holding bins (leaving just the sand in your tank).
• With the sand bed disturbed from removing the rocks, you will discard the remaining water.
• Then thoroughly wash your sand until it rinses clear (you can use tap water for this).
• Clean up your tank and equipment.
• Now just put everything back, adding enough saltwater to replace what was lost (which can easily be more than half of the original volume of water).

 

Note: Cyano can often be gotten rid of by using antibiotics like ChemiClean.  However, this won't resolve the underlying cause (mainly excess organics).  While I don't recommend going this route, it can sometimes be useful in conjunction with maintenance.