In any DSB low oxygen areas are often within 1 to 2 inches of the surface. Below a couple of inches, coarse crushed coral substrate gets just as anoxic as sugar sand. Without having a positive exchange of fluid, and I do not believe critters make that much difference in getting oxygen deep into a DSB, you will rapidly have anoxic conditions as the aerobic bacteria use up the oxygen in the bottom of any DSB; coarse or fine.
The Carib Special grade substrate I used in my tank is a uniform coarse crushed coral. There is plenty of life in the substrate. It is easy to see worm burrows from top to bottom and plenty of discoloration all the way to the bottom of the substrate where the substrate and acrylic meet in the front.
My snails continue to burrow just below the surface when the lights are on. The pod population is keeping my Mandarins fat. The whole bottom is densely cover with spaghetti worms. In fact, on several occasions I have watched the spaghetti worms disburse heavy white clouds of eggs into the water column.
pH is 8.3, S.G is 1.026, calcium is 425, nitrates is 0, nitrites are 0, and ammonia is 0. I have a small fuge with macro algae.
A few weeks ago the phosphates started to climb and reached as high as 5 after feeding the fish. The skimmer doesn't pull all the phosphate out. I do not believe any is being leached from the substrate. I have a dozen fish in a 60g system and feed heavily which is why the phosphates started building.
When the phosphate climbed I immediately had a hair algae bloom on the live rock. I started using RowaPhos in a bag. After a couple of days the Rowaphos started to solidify in the bag and wasn't very pervious to the water flow. But the phosphated still quickly dropped to 1.
I have since made my own fluidized bed filter to put the Rowaphos in and have now reduced the phosphates close to 0. The Rowaphos slowly rolls and boils at the bottom of the fluidized filter I built and no longer clumps. The filter sits in and over flows into my fuge. It was simple to build and requires little maintenance. Once a week I add a teaspoon of fresh RowaPhos in with the old while the filter is running.
All of the hair algae is gone, except in a few spots at the top of my live rock column where the live rock is breaking the water surface, just in front of the over flow where excess food can collect. The little algae at remains at the top has not completely gone away but has turned gray and is dying. My yellow tang should have it all the dying algae eaten within a couple of weeks. The algae in the fuge grows very slowly, indicating low nitrates and phosphates.
I am in the process of redoing my lighting to two 250w MH, a 20k and 10k bulb, to increase the intensity for the sps. The lighting is on a slide rack that easily moves it out of the way, so I can get at the top of the tank. The original lighting unit, (5) 75w VHO and a single 250 w MH, was very heavy and a pain to remove. *Any one want to buy 5-24"VHO's and a IceCap electronic ballast?
All of the coral, soft and sps are growing very well, now that the phosphates are getting close to 0. I get little to no algae growth on the tank glass or anywhere in the tank now. So if any phosphate is leaching from the substrate it can't be very much.
I plan on running an experiment in which I will slowly meter a small quantity of vinegar, using the CPW system, into the bottom of the bed. The idea is to increase the microbe population and drop the pH in the very bottom of the bed. I would like to see if it helps to put any phosphate plating out deep in the bed back into solution, so it can be flushed out with a water change or during the CPW wasting and eliminate the need for RowaPhos.
If it doesn't work I will simply keep using RowaPhos in the fluidized filter to keep phosphates near zero and the coral happy.