Can't grow Macros to save my life!!

[Cearbhaill]

Why is the world can't I get macros to grow in my refugium??

The refugium area of my sump is 15 gallon, 16x18x12, and is lit with a Coral Life strip reflector 28W 50/50 bulb. It contains rock rubble (and larger pieces) two banished serpent stars and one decorator crab. It has also had added various live mysid shrimp, pods, sand booster packages, etc. since the tanks setup.
Could one tiny crab be eating all this algae??

Since setting up this system 1-1-04 I have purchased four or five batches of chaetomorpha- all of which have slowly wasted away.
I have also purchased all colors of Tang Heaven macros- they also all wasted away. Even caulerpa disappears.

This refugium is fed by water returning to my display in a slow but steady flow. At first I was sure there wasn't water movement enough so I added a Maxi 400 powerhead to the fuge, got a new batch of chaeto- and it is wasting away also.

I am so stumped. Today I took off the 50/50 light and put on a regular old normal floursecent 40w striplight just to see if that helps. Should I maybe lay the spare one along one end to add more light?
I've tried 24 hour lighting as well as 12 on 12 off.

This mornings readings are

SG 1.025
CA- 450
Alk 11.2 dKH/ 4.0 meq/L
temps 81-83ºF
pH 8.1 AM- 8.3 PM
NH3- 0
NO2- 0
NO3- 0

Could my water be too clean (of nitrates) to support macros?
Is that even possible?

I have fish doing great, and all manner of corals (even difficult species) doing very well in my display. I do feed frequently (but sparingly), change maybe 10% bi-weekly, and skim heavily.

I am just stumped.
Everyone else seems to grow it no problem nd I can't even get it to live!
For more details of the setup click my red house.

Thanks for any comments or suggestions.