What happened? Should I worry?

[jmm]

One of my tanks is a 120 gallon butterfly tank. Besides the butterflies, there is one fire shrimp, some hermits and some snails. It has been running for about 4 years with no additions and no deaths. Up until just a month or so ago there was a lush coating of algae on the rocks and the glass would turn green in a couple of days after cleaning it. It didn't have any coral so the algae didn't bother me. It has a healthy population of pods.

About a month ago the algae started disappearing. The rocks are now completely clean (yes, completely). The sand is snow white and the glass gets a slight grey tint about 4 or 5 days after I clean it. It's amazing how clean it is. The fish are healthy, colorful, eat well, act normal and have clear fins and skin. They are growing.

The nitrate is zero. It always has been (two marine pure blocks). I use a protein skimmer and ROX carbon. It looks better, but it's never happened before and it mystifies me. Do you think there is anything to worry about?

 

[Vinny Kreyling]

I should have such a problem!

 

[mcgyvr]

Reduced feeding possibly?
Test for phosphates?
Any other changes?
Dose anything?
Test for iron?

Its entirely possible that nutrient levels or other elements are depleted to the point that the algae does not thrive anymore..

In a fish only tank thats really not something to worry about

 

[Tripod1404]

I assume your phosphate went too low. Do you use GFO or some other type o P absorbing media and recently replaced it.

 

[jmm]

I did drop a couple of Purigen bags in the filter socks not too long ago, but I've done that before without any diminishing of algae. And Purigen bags in my other (210 gallon) tank hasn't affected the algae. It's not out of control but it certainly isn't gone.