If you can get a microscope pic, that would definitely help to identify if it is dinos and then what type. There are some that are more toxic than others...some that are minimally toxic.
Elevate your nitrates to around 5-10ppm and phosphates around 0.06-0.10ppm. The GFO is not your friend right now...I'd stop it immediately. There are more threads on dino infestations after people use GFO and reducing Po4 too quickly. Do not let either of these nutrients bottom out...that's a recipe for dinos and cyano!
If a UV didn't work, then there's a good chance you are dealing with small cell amphidinium dinos. If that is the case...they are not very toxic and shouldn't have any effect on sand sifting fish, such as gobies. But, they are also one of the most difficult to clear out of a tank. There probably won't be a quick fix for this situation...best to expect a longer treatment for many months and learn to get excited by small improvements. I battled small cell amphidinium dinos for probably over a year until I won the fight...and i still get them to show up occasionally but in small manageable groupings.
Many will tell you to do a 3 day blackout...this won't work if it is small cell amphidinium. It took me about 9.5 days of blackout in a quarantine tank to clear this species out. And then they always seem to make a comeback somehow. This smaller blackout does works for some other types, though.
As you can see, you really need to know the type of dino you are dealing with in order to create an attack plan. One of my best reef purchases was an Amscope M150C microscope for around $75. I use if often.
Look up the Elegant Corals Dino/Cyano treatment recipe on R2R. This is actually the only treatment that worked for me to get control of my dino infestation...and I tried many, many things! But, you have to run it exactly as prescribed.
Good luck!
