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Barracks are usually a type of recirculating system. Instead of individual beanie type containers, a barrack is usually constructed of plexi and has individual cells to accommodate the fish. As water flows into the barracks it must exit and return to a sump. There are a few ways you can flow water through the system before it exits and that is what we will be discussing here.
Flow through barracks are designed to have water enter at one and and flow sideways through the cells before exiting at the opposite end. The first cell, where your water enters, is about 2" wide. Then the water will travel through cells housing bettas and then there will be a final 2" cell where your water will exit from. With this arrangement you can house 11 fish in 4" wide cells in a 4' long barrack.
The water can flow into the next cell in various ways. I have used a double partition that allows water to flow under one partition that reaches the top of the barrack and over a shorter one spaced behind it. You can also drill holes in the partitions to allow the water to flow sideways from cell to cell. What ever method you choose you will have to make sure you have adequate flow into the next cell, or you will not get the water turn over you desire.
If you chose to do the double partition, you need to determine the space under the first partition and also the space between the two partitions. Both of these spaces will determine the amount of flow you get through and into the next cell. The larger those gaps, the more water you can push through. I have found a 1/4" both under the first partition and between the two partitions to be good for flow. A 1/4" piece of plexi works nice as a spacer. Be careful if you decide you want more flow. If you get too large a space between, or even under the partitions, you get enough room for a betta to get down between the partitions and into the next cell if they are lucky. If you are not lucky you will find your betta stuck and often dead. Craft Popsicle sticks are handy to have on hand for just this reason. If you accidently get a space too big, and have some trouble with bettas getting in the space, a piece of cross stitched mesh folded over and slid down in the gap will keep fish out and allow the water and particulates to go through.
The height of the second partition will effect the flow of water into the next cell. That second partition is what will direct the water current up and across the cell. If you have a short second partition, around 2" in height you will find the current goes up and across the middle of the partition. Use a taller 4" partition and you will see the current go across the top of the cell. Both will have the current hit the opposite partition, where it will flow down and under the partition on that side. Either is fine to circulate the water in the cells. When you test your system out, add a little meth blue in the water in compartment and watch it circulate through the barracks. You will not be able to make any adjustments to that barrack because everything will securely glued, but it will allow you to tweak the next one you build.
The drawback to this type of barrack design, is when you clean the cells with a siphon, it will create enough suction to draw from neighboring cells on both sides. That will get the particulate matter you are trying to get rid of up and circulating in the water again. A barrack system will not get you out of siphoning feces and organic matter. The current does break things down a bit, and the finer particulates will head for the sump. But in time you end up with a large organic load and the particulates stay suspended in the water, not the healthiest for your fish. What a recirculating system with a sump WILL do for you is keep the ammonia and nitrite almost nonexistent and allow you to do change water on a lot of fish in a short period of time.
If you are concerned with disease in a recirculating system, this barrack design is actually nice for just putting water in and having it exit at the other end. That was my original intention with this design. I had a heated fish room and ran 20 gallons through the barrack every day. About three times a week I siphoned out the feces and uneaten food. The water collected in a large trash can and I just pumped it down a drain when I was done adding water. You still had to deal with the siphon creating enough suction to get your organic material back up in the water column. If I wanted to do a flow through now I would opt to drill holes in the plexi partitions.
Since a siphon will pull from neighboring cells and pull the material through the spaced partitions, I would not drill holes at the bottom of the partition but an inch or so above the bottom. And I would also be tempted to alternate between drilling at the bottom and the top. With out that second partition to direct the current up, you might not get the water in the cell circulating, If it enters from the bottom on the left side of the cell and has to go up and into the next cell, you get the water in that cell turning over much better than if the water just flowed sideways. And in the cells where the water would enter at the top it would be pulled down to travel into the next cell.
The next consideration is making sure you have enough holes drilled to ensure good flow. You don't want to be trickling water in because you you don't have enough holes drilled to allow a good flow of water through the system. You can err on the side of too many with no ill effect. Not enough and it may be a bit of a pain to get a drill into a cell and add more holes.
An overflow system has water flowing under a partition and over a other one behind it, like the flow throughs, but the water flows out the back rather then into the next cell. The advantages to this set up are each cell will get fresh water rather than water from a neighbor. Water flows out and into a back channel that will funnel the water to a drain. With the double partitions in this type of barrack, the height of the second partition will determine the water level in the barrack. I have mine at 5" in height. Bettas can jump so you do want a space between the top and your water level. You still want to have enough space under the first partition and between the two to get good water flow through the system. But again, you must make sure bettas do not get in this area. The 1/4" I have under and between will allow a betta to get real stuck. You can use cross stitch canvas again and slide some between the two partitions to block the passage of your bettas. I also use the 4" x 6" green pot scrubbies against the back wall of the cell. Water will flow through the scrubbie and stop particles from entering the the sump. And the scrubbie will also establish a great number of biological bacteria and assist in breaking down ammonia. And, best of all, keeps those fish out of the gap in the partitions. If you choose this option I still recommend having some craft Popsicle sticks handy because the fish always find a way to get super stuck back there and those sticks work slick to flip them out.
I feel this is a much better system for one that is recirculating.The fish don't get exposed to disease from their neighbors. Yes any disease would still be in the water stream