When you basically have a refugium in the display tank....

[BanjoBandito]

You gotta get in there. Pull the rocks, h2o2 it, basically rip clean it. There’s no product that’ll kill it off without causing other issues.

 

[rhostam]

I agree with the others. Nothing will deal with Merlin's beards of algae that have already grown in your tank. You must remove as much as possible by hand. If you can't remove the rocks and scrub as others have suggested, you can try other methods. Turning up flow/filtration and pluck by hand, using a hose and plucking while sucking it out, pluck while a friend catches with net, etc.

Only then can CUC help. There is also the issue of what is feeding the algae. You'll need to deal with that for long-term success. Remember that it probably took a while for it to get to this state. It may take just as long to fix the underlying issue.

UV will definitely help. Just remember UV sterilizes stuff in the water column to disrupt the reproductive cycles. It isn't doing anything to the water that will kill the algae already grown on your rock.

My band-aid UV from Amazon finally died and already I can see the algae on the glass and other undesirable appear on the sand. When it was working I only had to clean the glass about once or twice a month. Now, I'm back to daily (or more!) cleaning. I'm working on getting a properly sized unit installed in the next week or so. Additionally, I'm still addressing the high phosphate in my system resulting from my overfeeding. My problem is that PO4 is probably locked up in the rocks and is going to take a long time to draw it all out.

It's a challenge I'm dealing with myself. Good luck!

 

[Jib]

I haven't bothered testing nitrate and phosphate because they're likely zero'd out given the sheer amount of algae in the DT

This seems backwards to me. It might explain the dinos, but algae shouldn't be growing so out of control with nutients zero'd out. I'd test...

 

[Spare time]

Any ideas on how to combat this?

The system I had experience with that had this issue always had undetectable nitrates and high phosphates. Usually I like to say try and get things into the idea range and go from there. Lyngbya is a type of cyanobacteria. From my understanding, cyanobacteria is able to get nitrogen from the atmosphere and thus tanks with very low nitrate (or nitrogen in general) are unfavorable to other species but isn't an issue for cyanobacteria.

So I would try the following

1. Manually removed as much as possible by siphoning it out.

2. Then I would dose nitrate and phosphate to more ideal levels and keep them there.

3. You can see how things change overtime, or potentially add some other products that might act as competition. These would be things like waste away, eco balance (you can use eco balance with any of these), pns probio, microbacter clean, etc.. My idea behind this is that you make the environment favorable to other organisms, and then you add things that may kill or compete with the problematic algae/bacteria. Again, this is just a hypothesis, but it may or may not have worked for others who have tried it.

 

[jmNoles]

Update: Added UV 3 days ago and the dinos have, for the most part, gone away. It's mainly GHA now; small amounts of dinos will start to appear around 7-8 hours after lights come on and go away overnight.

Running Phosguard (read this was better for AIO chambers) and carbon with weekly water changes, and it appears the GHA is, ever so slowly, starting to turn white and die off. Next step is to add a CUC after my shipment from ReefCleaners was 90% DOA.

Still a long, long way to go. But for the first time in ages, I feel slightly optimistic about where this is going.