Finally, does anyone have any thoughts about using natural limestone boulders, in addition to live rock, to create the reefscape?
Reef rubble and limestone are both mainly composed of calcium carbonate, with other *stuff* mixed in the matrix. Some limestones are nearly pure, and are the source of the calcium hydroxide we call "lime" or "kalk". Other limestones have a large content of *stuff*. That stuff can include iron, magnesium, and phosphate. Beyond this observation, I can't contribute much to your decision. I could speculate, but in no way accurately predict, how this *stuff* will effect your individual tank as it slowly enters the water column. Besides, each of his have their own goals to meet. I like growing sandbed plants, so Seachem's "Grey Coast" calcite substrate is interesting to me because of its high magnesium and iron content which should be released in the low pH regions of a deep sand band. A different reefer would avoid Grey Coast (or a DSB) for exactly this reason. I will make a personal observation on using "base" rock as well as "live" rock. Over time, I have grown to resent all that rock in my tank, and regret putting so much in. IMO, "base" rock is a waste of precious space. If I were you, I would confine my selection to a few good pieces of live rock, cut to conform to the back wall, arranged as pleasingly as possible and secured in place. I guess in effect I'm agreeing with everything Mike proposed, and am just adding my personal perspective.
We've allowed some 25cm between the bottom of the tank and the start of the viewing panel to lay a base of live sand. Is this (25cm) adequate, too much or too little? Given the size of the tank and the fact that this is not a pure Berlin system, I thought we would forego a plenum. Does anyone have any thoughts on the subject. This seems to be one of those topics that not much is said about?
Um...is that last sentence intended as a joke? I hesitate to even bring up bottom substrates in threads anymore, because people have grown so passionate about them that bombs lobbed from the peanuts gallery tend to send the threads to hell in a hurry. Depending on your viewpoint, 25cm (10") of sand could be either a useful nutrient buffer where waste is recycled, or a stinking reservoir of filth. In my opinion, it will become both. Anyway, 10" gives you ample space to use a Jaubert (gravel) system with a plenum, or a just put fine sand straight down on the bottom. If you opt to not use a substrate or just have a shallow vacuumed substrate, I think that the water flow of the system should be designed to assist solid waste to flow to the sump/skimmer for removal, which lines up with what Mike and Idrhawke are encouraging. If you elect to use a substrate, I'd can list sources that discuss functional attributes like depth and constituency. My opinion is that you should pick a bottom substrate that is appropriate for the biotope you are creating. I keep a lagoonal tank, I have a deep sand bed in the tank. IMO, a fore reef or reef flat would not.
I can use either 1.5 or 2 inch polypropolyne pipe for the tank primary plumbing. This pipe uses a welding system for joining sections and fittings and is sold under then ame 'Vesbo.'
Sorry, I'm not familiar with Vesbo. If its non-metalic and rated for potable systems, its probably OK. My favorite rigid plumbing material for pipe over 1.5" diameter (non-pressure) is ABS. ABS is easy to get in larger sizes, sweep fittings are typical and cheap, and it can be adapted to PVC fittings using transition cement (a little tricky). For less than 1.5" diameter or pressure applications, the best pipe is flex PVC aka "SPA flex", IMO. The main idea is not to let plumbing resistance rob your pump power or flow capacity (gravity returns). So, big pipe diameters and soft bends (sweep fittings) are best.
Anyway, I assume a lower pressure system is preferable to a higher pressure system.
That depends on your purpose. If you are just recirculating water with the pump with a low head, yes low pressure. If you need to add energy to the water; like to lift it up 7 meters for a surge tank, drive eductors in the tank, or drive a mazzei to feed ozone to a skimmer (just can't get off the ozone!), you have to generate at least 10 psig.
turn the tank contents through the mechanical filtration system every 3 to 4 hours which implies an average load of around 5000 litres an hour. Any thoughts on pipe diameter??
Wow, thats really low for turnover rate, IMO. 5000L/Hr->1250G/Hr->20 G/min; did I estimate that right? That will not do! I realize that you want to keep electrical costs controlled on a tank this size, but you have to generate enough flow to "breath" the tank to atmosphere, and "breath" the organisms dependant on flushing flow with the tank. As a comparison, I turn my entire tank volume over every 1.5 minutes, and I'm still short of generating appropriate flow to model a lagoonal system throughout the tank.
My opinion is that you need to spend more time considering your water flow. I think that the lowest turnover option is to do what Mike suggested earlier, and go with a surged system to generate periodic random/high water movement. You've got what, 3000L->750G in the sump? Lets consider that you use 500G for a surge from an insulated tank(s) on the roof. Nothing fancy, just a bomb-proof gravity Carlson surge fed down with the biggest pipes that your overflows can handle. If you dump the surge once every 5 minutes, that's 100G/min->6000G/Hr->24000L/Hr for the return pump(s), right? More frequent surges would be "more better". Did you post how many linear inches of overflow you have? How much buffer volume (inches from the overflow to the tank top) do you have to help absorb a surge if you use a surge? Is spray or noise a consideration? Maybe you posted all this before and I missed it. Anyway, I don't think 5000 L/Hr will get you by, unless I don't understand what the goal is with this tank. But I'm not a big-tank guy, maybe I misunderstand the dynamics here.