The first thing that came to my mind is that you should be sure the caulerpa is a species that isn't toxic to fish. There are several species of caulerpa that are quite toxic to fish. The fish will still eat it but eventually become ill (common in an aquarium setting due to the limited food sources). They may be liking it, but it may very well kill them also.
I wonder if having my DT overflows going to the skimmer then the fuge is cleaning the water to much for the cheto to grow in the fuge.
If your skimmer is pulling out enough nutrients from the water going into your fuge to limit algae growth then it would limit algae growth in the same manner in the DT (unless you had an EXTREMELY low flow rate through your sump and skimmer.) With that in mind I wouldn't make any changes to your skimmer/sump/fuge set up based on that particular thought.
One potential problem commonly seen with increased algea growth (macro or micro algae) in a DT, while there is limited growth elsewhere, is if there is a significant amount of bound nutrients in the DT LR. Po4 can leach directly from the rock (or sandbed) fueling the algea growth in the DT while there is limited growth elsewhere. This would also likely accompany readings of very low po4 levels within the water column (even with photometers) since the algae is in close relation to the source and directly uptaking the nutrients as they're leaching out. Another situation in which this has occurred (at least referred to occurring in) is due to the precipitation of po4 when using kalk. The calcium phosphate molecules get bound up into the LR, substrate, or settle out and accumulate in a particular area. The thought is that the algea can either utilize the po4 within that molecule (breaking it apart in some fashion). I don't think any of that is proven yet (nor is much of the reefkeeping information/guidelines we live by) but it's been referred to as a possibility of causing localized algea growth when nutrient levels are otherwise low or undetectable.
Few thoughts for getting the macro and/or micro algae to grow where you want it to:
1) You may want to try changing your bulbs in your fuge. I've recently been studying up on algea turf scrubbers. 95% of the literature I've read about ATS's gives an indication to use CF bulbs with a K rating of 2700. Now, an ATS is quite different from growing macroalgae in a fuge, but the concept is the same. The algea thats grown on an ATS seems to respond best to the 2700 K bulbs and maybe the algea in your fuge will respond more favorably as well.
2) Not sure what you have for algae species in your fuge, but a bit of diversity can't hurt. I know you referred to having a species of caulerpa, but you may want to find someone with some fast growing chaeto. I'd be happy to give you some of mine, but I'm a bit of a drive away from you (I'm sure someone closer is willing to part with a bit of it for you). Various species of algae (micro and macro algae) are able to absorb nutrients at various concentrations within the water column. You may find a type of algea that will be able to grow and pull nutrients lower than what the caulerpa in your DT can withstand. For example, the turf algea that is grown in a well established ATS set up is said to be able to reduce po4 down lower than what common macroalgae's can tolerate.
3) If you think the problem is stored nutrients in you LR or substrate then only patience will prevail. Sorry. I have no magical answer for solving that problem. It's thought that nutrient reserves are most commonly found leaching from LR or substrate when the levels within the water column are quite low. I'm not sure if the comparison is correct, but I understand it as a nutrient moving from an area of high concentration to an area of low concentration to find an equilibrium. If this is the case, then a bit of time and continued good husbandry will be your best offense. Eventually the bound nutients become depleted and the nuisance algea will have nothing more to feed off of. What that doesn't solve is how the nutrients became built up in the first place. If you suspect kalk dosing could be the culprit then you might decide on another means of ca/alk supplementation?? You may have gotten some new LR recently that was from a system with high nutrient levels??? Poor husbandry in the past may have bound some nutrients into the LR or substrate??? Is there an location with poor flow that allowed detritus to build up?? In the same respect, if there isn't enough random flow or a sufficient CUC to keep the detritus off the rocks then it will build up in all the crevices and become a nutrient leaching pool. Tough to answer how the nutrients could have become bound up without more details to your system and the husbandry, but from your general description of having more algea growth in the DT then in the fuge leads me to stongly belive there are bound nutrients leaching out.
Good luck!
Keep us posted.
Jeremy
on a bad note its growing better in my DT then in my fuge lol.
