This is a concept I personally call "lazy man's dream fishroom." After dealing in last issues with water recycle technology which supposedly should  decrease the time we spend changing water and cleaning filters, we'll now present technologies that were developed for food fishes. Some of it is still applicable in our smaller fishrooms, and if we understand the concepts and apply them correctly, the right technology would allow us to save some of  the time we expend siphoning tanks.  

The waste produced by fishes and its feeding can be set at three categories: settleable, suspended or dissolved (Losordo et al., 1999). Settleable waste is the one that easily settle on tank bottom, suspended and dissolved waste hardly or never settle in a culture tanks, respectively. The levels of suspended and dissolved waste are reduced when you change the water and/or when water is passed through a mechanical filter and/or a biofilter, and settleable waste when you siphon tank bottom. For those running a flow-through system we'll present some tools intended help you create a so-called self-cleaning aquaria that should automatically flush all kinds of waste away from your rearing environment, but if you chose run a stagnant system this technology will deposit fish waste in a relatively small area where it can easily be siphoned instead siphoning whole tank bottom.     

The trick is to make water movement/current works for us due proper placement of water inlet flow and  outlet flow structures and/or aeration device(s). Particle settlement will take place where the forces which pull it down (gravity) exceed the forces which moves it horizontally (current), in short waste will deposit where water velocity is slower, usually they are the opposite corners to water inlet in rectangular tanks or the centers of circular and square tanks (Figures 1-5). If you place the water outlets at these points and if you provide your aquarias with flowing water the waste will be automatically flushed away. But if you run a system that is only provided of aeration its proper placement can create, at the above mentioned points, zones were currents are so weak that it allows the waste to settle there.  Also " For instance, in regions of low turbulence, the increasing probability of collisions between particles, compared to that which occurs in near laminar flow, leads to increased aggregation of solids and formation of larger particles. This is especially the case with organic particles, most of which have a greater tendency to agglutinate due to certain surface properties. In general, the formation of particles in aquacultural units probably depends on the same factors that affect suspended solids in natural freshwater systems, i.e., shape, porosity, density, inorganic, and organic composition and shear resistance of the particles in conjunction with medium parameters like turbulence and water chemistry. However, the stable condition of most environmental factors in fish farms means that solid formation is presumably controlled by a relatively small number of the more variable factors, specifically turbulence, solid concentration, and microbial colonization." (Brinker and Rosch, in press). 

Figure 1 - Tank configurations analyzed. From Oca et al. (2004).

Figure 2 - Velocity fields in horizontal sections with configuration 1. (h = water column height/depth). From Oca et al. (2004).

Figure 3 - Velocity fields in horizontal sections with configuration 2.  From Oca et al. (2004).

Figure 4 - Field velocities in horizontal sections of configuration 3 with two or three waterfalls.  From Oca et al. (2004).

Figure 5 - Field velocities in a  horizontal section of one of the tank halves with configuration 4. From Oca et al. (2004).

What Tank Length, Width or Diameter???

Our idea is not to fix specific values for tank dimensions because we understand that these factors are specific for each breeder condition. I mean that their rational setting  should take into account  parameters like mean number of fries per litter, desired stocking density, space availability and so on.  

Here we'll focus on the relationship between these values and its effects on the creation of water currents.

Under this topic the first question which reach my mind is "What tank shape is the best? Rectangular/square or rounded? Should I set a length and a width or just a diameter?" 

Circular Tanks.

"Recommended tank diameter to depth ratios vary from 5:1 to 10:1; even so, many farms use tanks with diameter:depth ratios as low as 3:1 and circular silo tanks use diameter:depth ratios on the order of 1:3." ... "Circular tanks make good culture vessels because they can provide a uniform culture environment, can be operated under a wide range of rotational velocities to optimize fish health and condition, and can be used to rapidly concentrate and remove settleable solids." Timmons at al. (1998). The water circulation in a circular tank assumes a flow pattern like the one represented in Figure 5 but it reaches much higher velocities, with same amount of  water being moved, due lacking of corners which can lock water.

The  self-cleaning ability of circular tanks has been pointed as its key advantage (Timmons at al., 1998). Water inlet of circular tanks is positioned close to its walls and water outlet at its center and the vortex created by water movement plus the absence of dead zones makes more solids moves towards the drain than should be expected to occur in a rectangular tank.

Like nothing is perfect the use of  circular tanks also represents a few problems. (1) I never saw a glass made cylindrical aquaria and while prices of acrylic tanks do not decrease the adoption of circular tanks represent more expenses, well unless you use your mother/wife Tupperware, plastic basins or buckets... ; (2) In the same floor or board area were you place an 1 m² square vessel you'll place a 0.785 m² circular tank, a loss of more than 20% in the amount of water stocked per square meter of floor or shelf board; and (3) Rearing unit design can effect growth, metabolism and behavior of fishes.

Ross and Watten (1998) studied the effects of the rearing-unit design (Figure 6) in the growth, behavior and metabolism of lake trout at two stocking densities. In general terms they observed that fishes reared in circular tanks shown poorer performance than fishes reared in plug-flow and cross-flow tanks (Table 1). Interestingly they did not clearly discussed about the inversely proportional relationship between current velocities (Table 2) and fish performance. I would expect that fishes which  lives in faster currents would have a higher energy demand and a higher metabolic rate than fishes from slow moving waters, but in this case like all treatments were fed a fixed amount in body weight basis sounds somewhat clear the reason because fast water fishes shown a worst performance when compared with fast water ones.

Figure 6 - Dimensions, flow characteristics and sectors (S1, S2 and S3) of the three tank designs. From Ross and Watten (1998).

Table 1- Percent biomass gain, food conversion rate and metabolism (ammonia excretion and oxygen consumption) of lake trout in three different rearing unit types at low and high fish densities. From Ross and Watten (1998).

Table 3 - Comparison of tank type (C - circular, P - plug-flow and X - cross-flow) and sector means for current velocities (cm/s) experienced by fishes at low and high densities.  From Ross and Watten (1998).

Accordingly Timmons at al. (1998), Losordo and Westers (1994) mentioned that "Water velocities of 0.5–2.0 times fish body length per second are optimal for maintaining fish health, muscle tone, and respiration." 

Nicoletto (1986) studied the effects of water velocities during growth phase of male guppies (until 2 weeks after they reached sexual maturity) on their physical condition, male signal intensity and female preference. The author placed newborn guppies in 20 gal aquarias (no measures specified) provided with a powerhead and with a clay pot in the opposite side to be used as refuge, the fishes were fed in excess. These power heads were regulated to move 1.5 or 4.5 liter of water per minute, the author mentioned that this higher  water movement rate created a current velocity that is close to the maximum velocity that a guppy can swim against in  laboratory flow chamber (~25cm/s). They observed that fast water males had significantly faster swimming speeds, wider caudal peduncles, were more attractive to females, had longer mean displays and spent more total time displaying than low velocity water males, but there were no differences in display rates, body widths, standard lengths or copulation attempts between treatments.  

The proper design and sizing of water inlet and outlet structures are critical to meet desirable rotational characteristics, mixing, and solids flushing. "Water injected into culture tanks through evenly spaced openings (holes or slots) in both horizontal and vertical injection pipes produces more uniform rotational velocities both radially and vertically, more uniform mixing, and better solids flushing."  (Timmons at al., 1998) (Figure 7).

Figure 7 - Water inlet structure recommended for a circular tank. From  Timmons at al. (1998) . 

On the other hand leaving water should be collected from tanks bottom and surface, at the same time, so carrying sunk and floating waste, respectively (Figure 8).   april2005  

Figure 8 - Typical double drain for removing settleable solids from a fish culture tank ( A = suspended solids flow stream, B = settleable solids flow stream).    Note: Water level is kept at the top of standpipe. From Losordo et al. (1999) .

Raceways. 

Raceways are relatively shallow rectangular tanks (see Figure 1 - Configuration 1) which should have a length to width to depth ratio of 30:3:1 for proper operation (Rakocy, 1989) but they are also built to meet 10:1 length to width ratio and ~1m depth.  These tanks has been used for both cold-water (Cain and Garling, 1993 ) and warm-water (Rakocy, 1989 ; McGee and Cichra, no date) fishes. Although the commercial use of traditional single pass raceways has been decreasing due constraints and risks like those cited in Table 1 by McGee and Cichra (no date) we mention it here because this is the food fish tank which closer matches the current behavior we have in the aquariums we already own and because we see that we could use the technologies which has been proposed to deal with its imperfections.      

The use of more than a single water inlet point will create a uniform current through the whole tank promoting  water mixing/uniformity like shown by Oca et al (2004), but will create low velocity currents outside the entrance area allowing the settlement of solids. Some raceways are intentionally built to have settlement/quiescent zones where solids are collected (Fornshell, 2001). These quiescent zones are located at tank ends where there are no currents caused by water inlet and the access of fishes to this area is limited by screens to avoid solids re-suspension.

Hybrid Designs.

We have noted that circular tanks offer the advantages of elevated water velocities, uniform water quality, and good solids removal characteristics whereas linear raceways make better use of floor space and facilitate harvesting, grading, and flushing operations (Summerfelt et al. (1998) page 253-263) so some improved hybrid designs has been proposed:

Cross- flow tank (Figure 6): "Water is distributed uniformly along one side of a cross-flow tank via a submerged manifold, and is collected in a submerged perforated drain line running the length of the opposite side. The influent is jetted perpendicular to the water surface with sufficient force to establish a rotary circulation about the longitudinal direction. Comparative production trials with hybrid stripped bass, tilapia, rainbow trout, and lake trout have been positive but application has been hampered by the need for the small diameter, fixed, and submerged inlet jets and drain ports, as well as costs associated with rounding the lower side areas to streamline flow." (Summerfelt et al., 1998) .

Mixed cell tank (Figure 9): "Here, a standard raceway section is modified to create horizontal counter rotating mixed cells with cell length equal to vessel width. Cells receive water from vertical pipe sections extending to the tank floor and positioned in the corners of the cells. Vertical pipe sections incorporate jet ports that direct water into the cell tangentially to establish rotary circulation. The pipe sections can be swung up and out of the water during fish crowding or grading operations. Water exits each cell through a centrally located floor drain. Hydraulic characteristics of the tank have been established and indicate that tank performance approximates that of a circular tank (mixed-flow reactor), both with and without fish present." (Summerfelt et al., 1998) . 

Figure 9 - Mixed Cell Tank. From Summerfelt et al. (1998)

Square tanks: Burley and Klapsis (1985) proposed an improved design of square tank which reached a circular tank like performance. We had no full access to this source but we suppose they had created something like a single cell like these we see in mixed cell tanks.

Tank Height.

Since most tanks should be designed taking into account a specific ratio between  length:width:depth or between diameter:depth we see that a small review on tank height and guppies could be useful. 

Most references I ever saw about the management of water column height for guppies were about the use of relatively shallow waters to ensure the survival of newborn fries, mainly for the so-called  weak strains. 

As far as I know the only one  who wrote something about the effects of  water depth on guppy growth was Elvis Bryant (1974). In general lines he recommended to use 20 gal high aquarias (18" height, ~ 45 cm) to induce body growth, and 20 gal long ones, with larger surface area, to promote caudal development.  Hann (1971), in another reprint also published in IFGA Extracts,  there is a table were I got that a 20 gal long aquaria should be 30" long X 12" wide X 12" high (75 X 30 X 30 cm).

Performing some calculations on the data presented at  Table 4 we founded that guppies bred in Singapore were kept in 30 cm water columns for  reproduction and male's growth phases. Females were raised in cage nets which were much deeper than cement tanks used to raise males, but I do not think it was used intended to promote female's growth but intended not waste expensive space raising cheap fishes.

Table 4 - The sizes of tanks and cage-nets, and their stocking densities at 10 farms in Singapore. From Fernando and Phang (1985).

For a larger image, click on image above

Aeration Devices Placement.

"Recent research has shown that the most effective placement for fixed electric paddlewheel aerators is midway along the longest side of a pond with the discharge of the aerator directed toward the middle of the pond. In this position, the aerator directs water perpendicular to the long side, developing circulation that reaches most areas of the pond. Placement of this type aerator in a corner of a pond and directing water diagonally across the pond provides poor circulation." Jensen et al., (1989) . Fortunately other researchers agree about the proper placement of aerators intended to create best patterns of pond water circulation and mixing ( Lazur and Britt, 1997; Boyd, 1998 and Hargreaves, 2003 ). So we simply present their recommendations for square (Figure 10) and rectangular tanks (Figure 11). 

Figure 10 - Paddlewheel aerator-driven water circulation used in shrimp production ponds. From Lazur and Britt (1997) .

Figure 11 - Recommended placement of a paddlewheel aerator in a pond to maximize water aeration and circulation efficiency. From Lazur and Britt (1997).

Our final thoughts. 

We agree that circular tanks can be taken as cheaper in the sense that: (1) these tanks demand that smaller amounts of water be moved to produce desirable water currents than other shapes; (2) rounded vessels have smaller superficial areas when compared with rectangular and square vessels holding the same amount of water which would decrease their manufacturing costs. Circular, square and 2:1length to width rectangular vessels, always holding 20gal (72 l) and 30 cm high, demands ~7609, 8281 and 8590 cm² of manufacturing materials, respectively. On the other hand these vessels waste physical space because they can't be tightly placed. This characteristic also allows that air circulate between vessels increasing their heat loss and heating costs. Personally as we are always concerned with space lacking in our fishrooms and because heating costs are usually higher than water and pumping costs  we should avoid their use. Between square and rectangular tanks we recommend square ones which can be cheaper and have better hydraulic characteristics.

Further References.

Bryant, E. 1974. Selection of Aquariums. The Darter, October 1974. Missouri Aquarium Society. In. IFGA Extract - Vol 4. p 126-127.

Hann, R. G. 1971. Constructing all glass aquariums. The Suburban Aquarist. In. IFGA Extract - Vol 3. p 1-3.

Losordo, T.M., Westers, H., 1994. System carrying capacity and flow estimation. In: Timmons, M.B., Losordo, T.M. (Eds.), Aquaculture Water Systems: Engineering Design and Management. Elsevier, New York, pp. 9–60.