Our filters serve two main functions, mechanical and biological filtration.  Mechanical to keep the water clear and biological to remove waste products namely ammonia which is highly toxic to fish, especially at a higher pH.  Two principal species of bacteria colonize our filters, Nitrosomonas, and Nitrobacter.  These convert ammonia NH3, to nitrites NO2, and then to nitrates NO3.  Ammonia and nitrites are very toxic and should always be immeasurable.  Nitrates are much less toxic, and are kept in check by water changes or in systems with large amounts of plants.

In a hatchery situation most people use bare tanks without any gravel or decorations.  This is the easiest to keep clean.  Filtration is usually handled by standard 4 inch sponge filters which do a very good job at both mechanical and biological filtration.  The purists will argue that these two filtration jobs should be handled by different filters with the biological filtration occurring last in the cleanest water.  Experience has shown however, that these little sponge filters at 1 per 10 to 30 gallons of water depending on the fish load will do the job just fine.  Cleaning consists of periodically rinsing them out in a bucket.  Not bad if you only have 10 to 20 filters, but a real job with 100 to 200 filters to clean.

The Filter
Due to space limitations I chose to use a fully contained sealed filter.  Another option would be to use filters open to air, but this would have required room that I did not have.  The sumps under the bottom tanks could be used as filter containers, but due to height limitations I can not access them unless I remove the bottom row of tanks.  I stole the idea for this filter from the new style of koi pond filters being designed.  They use floating polyethylene beads as both a filter material and to provide surface area for nitrifying bacteria.  They function in the opposite direction of a pool sand filter.  A sand filter would be an option but experience has shown that the sand cakes up too much after a little use and channeling occurs causing them to lose efficiency.  This doesn't happen to as great an extent in a pool environment, where the chlorine keeps the sand sterile and the amount of filtering required is actually much less.  Of course if you throw 8oz of flake food or koi pellets into your pool each day....  The filter I chose is copied from a commercially manufactured koi  pond filter.  I had access to one to measure and take notes, and was able to build it for less than half of its retail price.  For more details on the filter itself, click here.  

Cost for this system was probably between the cost of buying 2 to 3 sponge filters per tank.  Of course electricity costs are much greater due to the 1hp pump running 24/7.  This was a concession to allow much lower maintenance.  Cleaning of the filter system takes just a couple of minutes every couple of days.  The constant supply of fresh water also makes water changes unnecessary.  

Water Flow
Water is pumped from the filter via 2 inch PVC through the ultraviolet light and then through the heat exchanger.  2 inch piping is continued to each of the 3 rows of tanks on the 3 tank racks.  From here 1 1/4 inch piping runs over each row of tanks with drum valves placed over each tank to control the water flow.  Water overflows the tanks via standpipes and then through 3/4 inch plumbing run under the tanks and down between sections of the racks to sump basins underneath the bottom row of tanks.  I use Rubbermaid model 2550 50 gallon Roughtote storage bins that are cut down to 13 inch height, and drilled on ends as needed for 2 inch bulkhead fittings.  A 3 1/2 inch hole is needed for the bulkhead.  Each sump basin is connected to the next by 2 2 inch sections of PVC.  The last basin of each rack are interconnected via 2 inch PVC and drains water back to the pump.  Slotted PVC acts as a strainer in these last basins to prevent any fish from being sucked into the filter.  I also try to keep several koi (carp) in the bottom sumps as they constantly search for food keeping the sumps from accumulating a heavy layer of detritus. 

Home Up Ultraviolet Sterilizer Old Water Sump Fish Room Fresh Water Drip System Filter Room Fresh Water Reservoir Filter Heat Exchanger Air System