I second Manny's recommendation.
Adding a sand bed will only make a more difficult detritus/nitrate sink than a bare bottomed refugium. You need to siphon it of detritus, and you will not get a nitrate reduction benefit from it because there isn't enough volume/surface area in the standard refugium, just like he said.
That's why RDSB (remote deep sand beds, preferably in buckets) are the most elegant/efficient way to reduce nitrates without having to dose anything (carbon sources).
Mangroves are terrible nitrate reducers/uptakers. They need upkeep and do not do well with little light, and do not provide much benefit. They also soak up a TON of magnesium, throwing your chemistry out of whack, and don't provide as much positives as people think. People think that they provide a lot of benefits in our tanks because they see or know that they provide a lot of benefits in the ocean/on the coasts. It's benefits are all a numbers game. They're important in large numbers, as a habitat and as a (slight) nutrient uptake mechanism. In low numbers, they're not very effective at either of those duties.
Cheato or Caulerpa are MUCH more efficient reducers of nitrate and phosphate, but of course don't look nearly as cool or as pretty as a little tree. They still cannot do their jobs though if the nitrate levels are too high, as high nitrates will kill any macroalgae.
Maybe some rocks and some cheato, with a homedepot light over it. I wouldn't even use the shop light reflectors, just get a bulb that has it's own reflector built in, so it won't get dirty in the reflector, just on the outside envelope, which is much easier to clean.
I would LOVE to have a giant fuge area (150g+ breeder, long wide, shallow with low flow), as I love seagrass displays, especially since it would be almost a local habitat, since much of Biscayne Bay are giant seagrass fields....
One can dream, right?
On the subject of mud, it's great because 1. it's a dense substrate, so it works like fine grain sand with creating anerobic zones, and 2. It absorbs phosphate because it's soil/mud, with high iron concentrations in it, which helps bind phosphate. That being said I'm not a fan at all of it, since you will quickly lose the phosphate binder benefit (it must be changed, much like how you've got to change out GFO media to keep it working), and I'm not a fan of it acting as a denitrating sand bed, at least in 95% of the cases where it's used, because it's being used in waaaay too small an area to provide effective denitration, AND the fact that DSBs get gross in 99% of the cases, and have to be changed out eventually, as they stop working, and frequently leach back nutrients. This will happen especially quickly with smaller sandbeds, like the type you find in a split-chamber sump/refugium.
Shallow sand beds are fine if you want microfauna, but I wouldn't do a DSB or mud refugium.
Cheers
have failed with their refugiums to control nitrates. DSB work only if they are drained and cleaned. Mangroves are for show. Chaeto works very well with slow flow and high lighting. I dont know who's idea was to use the cheap home depot lights, but manny 
