Is this Dino's?
 

Are these Dino's?

I think it looks like cyano-bacteria to me.:question:

You mean dinoflaggeletic algae? If so I would say yes, how old is the tank?

I have been battling this reoccuring brown string algae for over 3 months now. I have syphoned the sand bed and blown off the rocks numerous times only for it to return within days. 10 days ago I syphoned out the sand bed and took pictures April 26 and tonight. Stuff is on rocks and glass as well.
Other characteristics:
- Snails become lethargic, are constantly falling off the rocks and eventually slowly die
- little air bubbles sometimes develop
- no nitrates or phosphates according to salifert test kits
- alk 9 dKH, cal 420ppm and mg 1400 ppm
- tank is 13 months old
- 3 days of complete darkness did very little.

Or is this just really bad diatoms?
Any ideas?

I had dino also. I got rid of it by not touchin it and cutting doen my lights to only 4 hours a day for one week and I also dosed with Seachem Stabillity to boost my good bacteria to help fight the bad. Havent seen it in 6 months now

Had the same algae problem as you pretty sure it's dino's. I did more water changes, lots of bastering and started on those NP pellets, after a month no more dino's.

I have done similiar. About a month ago I started
- Brightwell Microbacter 7
- n/p pellets
- increased carbon
- boosted p/h above 8.3
- stopped my CA reactor (to increase ph)
- shortened my photoperiod to 4 hrs

Still nothing...

That's very odd, well at least your on the right track. What are the tank dimensions and what kind of flow are we looking at?

120 G - 4X2X2

2 mp40s at 80% on reefcrest mode

Have you tested for silicates... in your aquarium and in your tap water/RODI water.