So I thought I'd try and explain a bit about my concept for this tank. The whole BB thing has exploded lately and the case for BB over DSB is certainly logical. However, pristine white sand just looks great and the bottom of BB tanks can look a bit dirty in my opinion. The faux sandbed looks great but after six months it won't look like sand. It will be purple or brown which kind of defeats the purpose of making a bottom that looks lik sand. So I know I wanted sand but I also wanted BB. The system I decided on is not new and as I worked on it I found the OregonReef employed a similar method.
The idea is to have sand in only the viewable parts of the tank (in front of the rock). The area which houses the rock is BB and makes up at least 2/3 of the tank. I'm going to plumb in a closed loop on the bottom with two 1.5" bulkhead outlets on each side where I'll build the rock structure. I'm going to make rock racks out of PVC pipe and fix them into the bulkheads. Each rack will have about 10 3/4" locline nozzles and the flow through these will keep the area under the rack and between the rocks free of detritus build up. Here is a picture of one of the racks that Steve uses on OregonReef:
One thing that concerns me is potential sandstorms from the flow out of these racks. I decided to create a sand barrier that winds around the length of the tank where the rock will go. I may not have to implement this solution if I can just build up rock to make the barrier but no harm in being prepared.
The Mold:
What I did first was build the tank out of plywood and 2 by 4 and filled it with earth. I then sculpted out the shape of the rock structure that I had in mind. When I did this I still thought I was going 60" long but have since decided on 72". Anyhow, I can widen this barrier thing no problem.
Aragocrete:
I learned about aragocrete on Garf.org so tried to mimic their aragocrete recipe. Basically works out to be a 5:1 ratio of media to cement. I used:
1 x crushed shells (collected from the beach)
1 x crush coral and plastic shavings
3 x sand
I poured it in and didn't make enough!:
So another trip to the beach later:
The last pic has the back of the tank at the bottom and the racks will sit in the square area behind the barrier. The sand will be placed around the front (about 1"). The aragocrete came out a bit more grey and concrete like than I hope. Like I said, I might not use it if I can get the same effect with just live rock. If I do use it, it will soon colour up with coraline. I have it soaking in water now where it needs to stay for 2 months. I broke it up into pieces too to help the leach cure process.