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  1. #1
    Moderator badflash's Avatar
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    Calculating your Bio-filter

    The first thing is to figure out the surface area of your bio-media. 1/2" pea gravel is about 90 Ft2/Ft3. Scrubbie pads are about 400 Ft2/Ft3. When mature, a bio-filter can remove about .05 grams of ammonia per square foot (Ft2) per day. To calculate how much ammonia you can remove, multiply the total surface area times .05. That means 1 cubic foot of scrubbie pads can remove 400 X .05 = 20 grams of ammonia per day. 1 cubic foot of growbed made from pea gravel would remove 90 X .05 or 4.5 grams per day.

    To figure the bio load from the fish you need to know how many pounds of fish you have and how active they are. A good estimate is about 5 grams of ammonia per 100# of fish per day. Over estimate your poundage as fish GROW!. Be sure you have more removal capability than you have supply. A safety factor of 2 should be considered minimum.

    Next you need to flow the water to the bio-filter fast enough. Ammonia levels will build up until the amount flowing into the filter equals the amount supplied by the fish. I'll do the calculation in liters because it is easiest that way. You can always convert to gallons later if you want.

    Suppose you want to limit the ammonia levels is less than .1 ppm (parts per million) That means you will have no more than .1 gram of ammonia in 1 million grams of water. 1 million grams of water is 1000 liters. If you want to remove 5 grams per day you must flow at least 50,000 liters per day through your filter. you get that by taking 5 grams of ammonia per day and dividing that by .1 grams of ammonia per 1000 liters of water. To get liters per minute divide that by 24 hour/day X 60 Minutes per hour or 1440. That gives you 37.7 liters per minute. This can be converted to 10 gallons per minute.

    More fish means higher flow required.

    The end product is nitrates and phosphates which must be removed by using plants or water changes. The thumb rule of having 2X the grow bed area as you have fish is based on having enough surface area to support the plants needed to absorb that. It is a VERY rough rule and depends on the plant and its needs. A heavy harvest will disrupt this, but not the bio-filtration of ammonia.
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  2. #2
    Moderator jackalope's Avatar
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    Re: Calculating your Bio-filter

    Thanks for that info badflash, I'm working on a bio-filter for my setup to see if I can't improve on the growth and budding of the plants (I'm not sure it will help, but it surely couldn't hurt )

    I have a question about the calculations ......

    1. Fish size & species ...... you might have two 1 1/2 pound Tilapia (3 lbs) that you're using for figuring, where I've got maybe 100 feeder Guppies (1/2 lb?) ...... your Tilapia process the food at a much higher rate (1.2:1) than Guppies which probably don't have a ratio listed that we can use for a figure ......... so even though you have more poundage of fish, I'd venture to say that my fish would put out more ammonia/waste than yours. Is my thinking on this correct, or am I all wet?

    It doesn't have to be Guppies, it could be catfish, Bluegill, Crappie, Bass etc. they all convert the food at different rates, so wouldn't that make a difference in the calculations?
    (i.e.TILAPIA convert food at around a 1.2 to 1 ratio, Meaning for every 1.2 pounds of food they eat, they will grow 1 pound. Catfish and Bluegill ratios are around 4 to 1, Crappie 6 to 1, and Bass are slow at around 10 to 1.)

    I just wanted to clear up this question before I finalize my figures for the filter I'm making . THX
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  3. #3
    Moderator badflash's Avatar
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    Re: Calculating your Bio-filter

    The number I put out of 5grams per hundred pounds is an average. That is why a safety factor is needed. Tilapia run in a range of 4 to 10 grams per 100#. I don't have specific numbers for other fish, but I'll see if I can find them. 5 is a conservative number for any other fish as their metabolism will be much lower. Metabolism is the key. Higher metabolism = more ammonia. Tilapia have a much higher metabolism than most other fish. Later I'll post some info on how to estimate ammonia production based on food intake.

    Goldies have special issues as do tilapia. They make a lot of poop and that must be removed as well as the ammonia. The great thing about a growbed is that it is a great particulate filter. Using a flood & drain system with composting worms allows for the digestion of the solids.

    The calculations are meant as a guideline, so test your water. The numbers will show that you don't need much of a bio-filter for ammonia. Nitrate, phosphate and solid removal is why you need large beds.
    The best fertilizer is the farmer's shadow

  4. #4
    Moderator jackalope's Avatar
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    Re: Calculating your Bio-filter

    I did away with about 1/3 of my system (the gurglers), so I'm going to replace it with a biofilter ...... when I get it ready to put together, I'll take pics and post it after I test it to make sure it will work the way I think it will . I can't go any larger on the growbeds right now, so the biofilter is the only answer for now.
    I use the Linux Operating System ...... Free as in beer!
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    Aquaponics - food'n'fish at your doorstep

    Helena, Montana - Home of the Northernmost Monument to the Confederacy

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