I don't have any exciting updates to add. Here's a brief description and summary of a Duplex system I set up about two years ago. The components are listed in the order in which the water flows through them.
Display size 100 gallons
Settling drum 55 gallons
DSB & live rock drum 55 gallons
Frag tanks size 120 gallons
Duplex filter size 120 gallons
The idea was to add lots of system volume to a moderately small display tank. The client had room in the basement where the water can stay cool and the filtration devices can be spread out where they can be easy to service.
The display tank drain has one 1" siphon drain with a 1" emergency/excess durso drain. The long drop to the basement offers excellent siphon power. I probably should have used a 3/4" or 1/2" siphon line for better siphon stability and easy siphon start/prime. As the water drains into the 55 gallon settling drum, detritus sinks to the bottom as clean water overflows near the top... well that was the plan. After a year and a half the drum didn't collect anything significant. Instead it was chock full of feather duster worms and tunicates (sea squirts), and sponges. I was hoping to use the drum for water changes, as it has bypass valves so the system still runs while the detritus and water is removed. Once the new water is fully mixed (salt) and aerated in the drum, it can be turned back online. Like i said, that was the plan, but it was always clean. Either the display didn't generate enough detritus, or the turnover was too slow (+- 450 GPH with a Poseidon T3 pump) to collect detritus, or the detritus was too light to sink to the bottom. I think the problem was the latter. Perhaps a labyrinth of some sort would help separate the heavier detritus from the water???
The system has zero nitrates and phosphates but I can't say for sure what the deciding factor is for that. The protein skimmer is a premium model (Bubbleking) but it doesn't collect a heck of a lot. It usually just fizzes away with wet foam below the top of the neck. The benthic zone would accumulate detritus about every six months. The top eggcrate panels are removable for siphoning. The detritus comes from the refugium above. In the future I would consider using a shallow food grade plastic tray to hold the macro algae (chaetomorpha). A series of smaller trays would be easier to manage. One tray could be cleaned and harvested weekly. When you cut or tear macroalgae it "bleeds" nutrients into the tank along with secondary metabolites that are mildly toxic. By completely discarding one tray of algae without tearing or disturbing it, water quality and stability is improved.
A few larger starfish and urchins would help keep remove some of the detritus from the benthic zone. The benthic zone itself was typical of other ones I have built. A network of eggcrate about 6" tall by 30" wide by 48" long. The first few partitions of eggcrate were more populated with critters than the last half. I think there is a benefit to seeding the last half of the eggcrate with small rocks or collected benthic inverts. As far as detritus removal goes, it might be a good idea to have a slight slope on the bottom panel so detritus migrates to the end for easy siphon removal. Another idea would be to mount a powerhead at the bottom at one end (at the start) and plug it in for five minutes every week. It could even be on a daily timer. It would be just enough to "stop the dust from settling". It would also help stir up plankton (pods) and deliver it to the fish and corals waiting in the display tank.
The DSB drum has passive flow running over the surface at a rate of 450 GPH (from settling drum). It appears to be detritus free but I haven't taken a close look. Like the settling drum, it's dark blue plastic with a black lid and no illumination. I can't report if it has a positive or negative effect on water quality, namely nitrate reduction (denitrification), but in theory it has a large dark anaerobic zone with a constant passive supply of new water at the surface. If I were to do it again, I would add a series of 3" diameter perforated PVC tubes with an end cap at the bottom. I would push the (maybe 10?) tubes vertically into the sand and leave them empty. The tubes would provide some additional water exchange to the sand bed as well as provide easy access to add a sulphur or carbon source. Temperature and dissolved oxygen can be monitored by dropping probes down the tubes. A heater at the bottom of the tube will cause thermal exchange as hot water rises in the tube and cooler surface water is pulled down into the sand bed. I think something like this will greatly improve efficiency without compromising the anaerobic environment. Alternatively, I would use about 25 tubes and fill them with sand. This system would add more surface area and perhaps increase efficiency. Water would be able to passively flow throughout the empty areas around the vertical tubes. A carbon or sulphur pellet could be added to the sand mix in the tubes. The tubes could be periodically removed to evaluate if hydrogen sulphide is forming (black sand and rotten egg smell), detritus is collecting (which isn't likely), and if carbon & sulphur media is dissolved. This kind of sand tube configuration would be a compromise between live rock (less surface area) and a deep sand bed (more surface area). What it offers that these two do not is more interface surface area without losing the anaerobic zone.
The frag tank got completely overgrown and ended up with more coral (mostly sps) than the display tank upstairs. In the end the bioload was quite high so the idea of increasing system volume wasn't fully realized. If there was more room, a daisy chain of 55 gallon drums would be a cost effective way of achieving the original goal. Huge amounts of system water volume results in a low bioload with the appearance of an overstocked display tank. The system stays cooler, water chemistry buffering is more stable, and there is room for nutrient levels to rise a little. A bypass drum is a convenient and safe way to change water.
I've been leaning toward more light for the refugium. As much as chaetomorpha grows well under low wattage compact power twist fluorescent bulbs, growth and subsequent nutrient export is exponentially better with T5 or metal halide lighting. I don't think LED lighting is ready for market yet, certainly not the mass produced units from China. The intensity and light spread are not quite up to... you know... par
If you feel compelled to use LED, don't use the blue bulbs as they serve no purpose for algae growth. As I've mentioned many times in this thread, the algae should be kept in a shallow 4" tray so all of it is illuminated with minimal shadowing. It also assures that the lower half doesn't slowly degrade and fall apart, releasing nutrients back into the water. Another possible modification would be to use a timer to take the refugium offline during the day while it is in the dark (reverse photo period). Photosynthetic respiration occurs during the dark period as the algae converts oxygen to Co2, driving down PH. During (dark period) respiration, the algae releases a portion of its nitrate, phosphate and heavy metals. By taking it offline in the morning when the refugium lights go out, and turning it back on at night when the refugium lights come on, it minimizes nutrients leaking out of the algae and returning it to the system water. You could use the same timer to run the refugium light and the pump that supplies water to the refugium. The benthic zone is non-photosynthetic so it does not have this nightly problem.
If I had the time and resources I would test the day and night nitrate and phosphate levels in a system with a large refugium. If nutrient leaking can be confirmed, a daily shut-off system would be worth the trouble. If the fluctuation is minimal, such a system wouldn't be worth the hassle and risk of timer/pump failure.
One of the far-fetched but interesting ideas I mentioned earlier in the thread is the concept of keeping a significant bed of large (Hawaiian) feather dusters in a benthic zone. The feather duster worms collect detritus out of the water column along with heavy metals, nutrients and other "bad stuff". Every week you could remove a dozen or so tubes and discard them without too much work. Within a few weeks the tubes would be restored by the worm without expending excess energy. They are filter feeders so they polish the water as an added bonus. If we had quantitative numbers of the content of the soft worm tubes we could decide if the practice of farming and exporting them is viable. Various worms (earth worms, tubifex worms etc.) are used in solid and liquid waste management on many levels. This idea is just an extension of that practice.
We also talked about clam or scallop beds earlier as water polishers. Phytoplankton and even adding diatoms would help feed the bivalves (clam-like critters) but the nice thing about benthic zone organisms is they thrive on whatever floats by with no nutrient import (feeding) required specifically for them. The problem with clams as a filtration method is they require illumination that uses energy and heats the water, maybe even encouraging nuisance algae. Clams are also a drain on the calcium and carbonates in system water. Scallops are preferred because they are non-photosynthetic, requiring no light, but they are notoriously difficult to keep (unless it's just me). They tend to get skinny and slowly whither away. Oysters seem to be more hardy and are also non-photosynthetic. They are more of a thing you find attached to a coral than something you buy on its own, but maybe we should be focusing on a colony of oysters in a benthic zone.
If you were able to read this far you must have some info to share on these ideas
I would love to get some feedback on non-photosynthetic bivalve longevity in reef tanks.
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