I have a Nanocube (24G) that has been established for 2 years. For the last 6 months I have been fighting what looks like a major diatom breakout.

I have had two percula clownfish in the aquarium for 16 months, and I've reduced the live rock down to about 10 pounds. All my coral & live rock was moved to other tanks which have been established for about 6 years. I have turbo snails & hermits as a CUC.

In a period of 24 hours I get a huge covering of what looks like diatoms over the rock & sand. Each morning by 9 am I am in the middle of another diatom breakout. I have a diatom filter and I am cleaning the tank daily (turkey baster) to keep the algae under control, but I'm at the point of thinking about restarting from scratch with this tank.

I've changed bulbs & tried a different light source with no success. I have an RO/DI filter and check each batch for TDS (Zero).

Water is 80 degrees, SG is 1.024, no problems with water chemistry tests (mine or LFS). Flow is a Maxijet 900, and Koralia 1. I have chemipure elite & polyfilter in the back chambers. Water change of 10% weekly. I used to run Chaeto, but I couldn't keep it clean.

Any ideas? I'm running out of patience with this tank.

Pictures would help. I do think you have dinos not diatom. Which would mean more of a bacteria problem. Dinos is a pain in the butttttt!

What are you phosphates at?

Give us all readings you are able to test for please.

I agree with Nick that you should check your phosphates (if you haven't already). If phosphate is running anything close to .01, I would decrease feeding and increase your water changes (R/O water I assume) to perhaps 20% weekly. You might consider adding a phosphate reactor with ROWA-PHOS which is great stuff. I hope you can get this under control....

I'll run the full list of tests and advise.

An interesting problem, dinos could certainly be it, and phosphates could be contributing, but for the sake of figuring it out here's some random thoughts which may or may not be useful:
Less live rock means less filtration capacity, this could be linked, especially that you're running less than 1/2 what you'd expect in a tank that size.
Any macroalgae in the tank? Perhaps chaeto won't work for you, but having something in there to soak up nutrients could definitely help.
You mentioned the sand, but is it crushed coral? Is it coarse sand or relatively fine? A vacuum could be in order.
Also the time honored 3 days without light could improve the situation, but I've never really been a fan of that option.

DINOS!!!!

If it's Cynao (redish bacteria) that is typically called an algae it usually stems from either phosphates or alkalinity being too low. I had a small out break from over feeding which jumped my Phosphates up. I did 50% water change over a week, vacuumed the sand, and put in one of those phosphate remover bags in my filtration. The Bacteria was gone overnight.

I don't suggest doing this until your tests are done and you confirm it is in fact Phosphates.

Ok. Here's the results of the tests -

Ammonia 0
Nitrates 0
Nitrite 0
PO4 0
pH 8.2 (5pm)
CA 370
KH/ALK 8.8
Temperature 80 deg

I've looked at all the pictures of dinos and I'm not sure. I don't see any stringing or bubbling.

Even with lights out (overnight) I'm still seeing some growth. I'll maybe try lights out for a few days and see what happens, unless someone has a better idea. I may start dosing B-Ionic to try and get the CA & ALK up.

I'd say try and raise your Ca and Alk but that probably isn't the culprit. Do 20% water changes...."the solution to pollution is dilution". Cut down on your feeding if possible, and try and give cheato another shot.

I've had these types of outbreaks in my 1-1/2 year old tank. It looks like coca-cola has settled on the sand bed and if I turn off all circulation for a few minutes the algae starts to turn 'stringy'. They specifically occur if I aggressively stir up the sand bed or clean out the sump after an extended period of detritus accumulation. No amount of ChemiPure or any other substance seems to have any effect. All my testable parameters are always 100% normal for a reef tank when an outbreak occurs.

I suspect these are dinoflagellates of some type that feed on specific substances that are made available when detritus is stirred up and distributed throughout the tank.

I have found the best course of action is to thoroughly clean out detritus pockets wherever they occur and reduce coral/fish feedings. Often it takes 3-4 weeks to clear this scourge, but sometimes it takes longer.
Turning off the lights for a few days doesn't seem to effect the algae much in the long run since they usually come back within a few hours of the lights being turned on again as long as their food source is still available.

Good husbandry and patience will eventually clear this.

Agreed. Substrate sources of P are usually to blame for most cyanobacterial films. You can have negligible concentrations in the water column, yet have the typical mats because of this.

My understanding is that phosphate is present in a reef aquarium in different forms. The one form we typically measure is inorganic phosphate (aka 'orthphosphate') and the other major form is organic phosphate . When detritus is stirred up, we are making available a large quantity of orthophosphate to naturally occuring algae and symbiotic bacteria in/on the substrate which secrete phosphatase enzymes that liberate the usable orthophosphate from the organically bound phosphate. Add a light source and dinoflagellates then enjoy the bounty of liberated orthophosphate at the substrate level and use most of it up before it reaches the general water coloumn. That's why phosphate can test at '0', but dinoflagellates can prosper. The process of phosphate conversion seems to be rather slow, but steady, thus the dinoflagellates can persist for weeks and even months.

Keep in mind that "substrate" is used in the broadest sense in that it refers to whatever the cyanobacteria, etc. are sitting on directly. This can range from detritus deposits, P enriched carbonate sand, P enriched live rock, etc.

The process you describe above is certainly true of how various organisms are able to liberate more usable orthophosphates from organic molecules. Stirring the sand will also solubilize bound orthophosphate, which is immediately available.

Do you have a sump of any sort? Is detritus collecting in the back chambers and I agree vacumming your sandbed may helps, in an established tank with no detectable phosphates detritus is probably collecting somewhere and may be the source.

Yes, I intended 'substrate' in the broadest sense as you stated.

I worded my post incorrectly. I meant to say that "I" meant substrate in the broadest sense. I knew you did from the start, but I was trying to clarify myself.