Looking for suggestions

Sorry this is so long.  I wanted to give as much info as possible.

• 20 gallon glass tank
• 5 gallon sump (tupperware tub under the stand) - not a refugium
• 2 x 55W Power Compacts run 8am - 5pm
• 15 lbs of live Fiji rock with some nice purple coraline
• 2 inch sandbed seems to have definite layers (i often vacuum the top but have never disturbed the deeper layers)
• 5 hermits, 3 emeralds, 5 astreas, 5 nessarius, rose anemone, a few zoos, a few star polyps, fingerleather.
• a few clumps of calerpa macro algae growing in the main tank, probably not enough
• specific gravity: 1.025 (Instant Ocean salt)
• alkalinity: 4.5 meq/L
• pH: 8.4
• nitrates: 0 ppm
• ammonia: 0 ppm
• phosphates: (waiting for test to get here!)
• 78 degrees seems to hold steady
• I change 20% of the water every 2 weeks
• RO water (have tried from 2 different sources)
• dose every once in a while with Salifert "All in One"
• feed frozen brine shrimp (about a pea size a week since i don't currently have any fish).

This tank ran well for almost a year with fish, shrimp, corals.  For the last 6 months, I have had increasing problems.  For one, cyano and green hair algae increased over a couple months and have maintained a steady rug.  I only had a few fish (clown, scooter blenny, leopard wrasse) but they all died within a 2 month time frame.  I also lost a previously healthy frogspawn and xenia (shriveled to nothing).  My zoos, fingerleather, star polyps are puckered up and stopped spreading.  A couple dozen asteas have dropped dead.  The anemone seems to be fairly happy, but i've seen her (yes, her) happier.

All very tragic, but being the cheap and busy person that I am, it all just happened.  I continued to do water changes and changed the SeaChem SeaGel (phosguard+carbon) pellets every week or two for several months, hoping to ride it out.  I took water down to the local fish store and each time they did a few more tests and said things looked good.  They recommended the SeaGel and mexican turbo snails (died within two days).

I've decided to repent and figure out what's going on.  Since I had lost all livestock and the algae was out of control, I decided to start with a complete water change and some manual algae removal.  I mixed up/heated a big tank of RO water.  I used my wife's toothbrush to scrub the hair algae from each rock while in the tank.  I shook the detritis from each rock and moved the clean rock directly into a bucket of prepared water.  Once all rocks were out, I syphoned all the water/algae out of the tank until the water was at sand level.  I then "vacuumed" the top 1/8 inch of sand out tossed it.  I then filled the tank with the new water and moved the rocks back into the tank.  I added a fresh bag of SeaGel.

It looked great for a day or two.  I acclimated/added the livestock mentioned above (snails and crabs).  Half the astreas died within a day.  The nessarius and crabs seem to be doing fine.  I pulled out the dead astreas as soon as i was sure they were dead.  The hair algae and cyano is already coming back 3 days later!  I got some tests (results listed above).  I know that phosphates are the typical culprit and I'll post that when i get the test done, but i'd think a full water change would've ruled that out at least for a while.

Where do i go from here!?  If it's phosphates, what could be causing such a sudden flood of it?  Any other thoughts?

joellyons wrote: I know that phosphates are the typical culprit and...   Any other thoughts?

With all due respect, I'll just say that your knowledge needs to be updated. I cannot write nearly as well as I can talk (and even that is suspect). Feel free to call me at the number below and I will show you how to resolve this problem.

Add 2" of oscilate sand. Add a skimmer. Add Macro algea behind the rock work where it will get light(Ill give you some)or better yet turn your sump into a refugeum. Do your next water change with LW from someone elses tank. Get some LS from someone to boost your bio diversity. I doubt that Phosphates are your problem. I dont know about Sea Gel. I would pull it out and add a nice size bag of carbon . Then leave it alone for a while other than water changes, and give it a chance to balance out.

Just my 2 cents. Been there done that. Wish I haden't. 

A few minutes on the phone with Joel and my brain was picked clean.  

If there was a small and also effecient skimmer available, it would probably sell very well for nano tanks. But in the absence of that, using Activated Carbon with some kind of floss to filter out particulates is the best option. That's why all the smaller tanks (10 - 40 gallon) I set up use a HOB filter with AC.

Alternatively, I run a CLAM: