Grow bed size calculation

[Brian Hillestad]

I have a 100 gallon fish tank now and small (2-2x3) grow beds.  Things are flourishing.  I want to increase my grow bed size to the maximum I can get on my 100 gallon tank.  Is there a formula for maximizing grow bed size for a 100 gallon tank?  I have two 26" x 5' long tubs that I could use if I can make them both work.  Right now I am using media based beds, but interested in doing 1 media bed and 1 raft bed to compare.  Any advise I can get would be appreciated.

[Sylvia Bernstein]

Hi Brian.  As a general "rule of thumb" you want to have a minimum of a 1:1 grow bed volume to fish tank volume, but you can take that out to 2:1 or 3:1 if you have some way of ensuring that your fish tank doesn't get drained on a flood cycle - like a sump tank (I recommend studying Murray Hallam's Chop 2 design for this) or an indexing valve.  How deep are your grow beds?

[Brian Hillestad]

Thank you.  My current beds are 8" deep.  I am growing tomatoes, lettuce, cabbage, and swiss chard.  I was thinking of introducing a raft system after the water had passed through my media bed to remove solids.

[Sylvia Bernstein]

You might want to check out the Hybrid Systems group in here to ask them about adding on a raft bed.  I know Sahib has done it successfully, and I also know that he does a bit more solids filtration after the media bed.

[TCLynx]

If you can set up your system such that the fish tank drains by SLO (Solids lifting overflow or venturi drain) to your media beds and the media beds could drain into a raft bed where the pump could lift the water back to the fish tank you could have quite a lot of growing area.  Then the only question is balance.  You may find you run out of nutrients if you have everything planted heavily and only a small population of fish or you are not feeding high quality/protein feed.  Then again, there are so many things involved in the balance that I've experienced some systems heavily planted out with still too much nutrients and other system where I can't figure out where all the nutrients went.

 

Anyway, for the basic answer to the question you were probably meaning to ask.  To estimate how much water it takes to flood a given container, plan on about 40% of it's total volume.  So if you have a simple system where the pump is in the fish tank and there is no sump or indexing valve, from a 100 gallon fish tank you could probably manage as much as 100 gallons total worth of flood and drain grow beds provided you stocked the fish tank as if it were only a 50-60 gallon tank (since the water level will fluctuate.)  Of course you can add constant flood grow beds or raft beds pretty easily without worrying about water level fluctuations and the only question then becomes, do you have enough nutrients to feed the added plants?  I would say don't worry too much about it, if the nutrients drop off just stop planting or get more fish/tank/or higher protein feed.

[litano]

I am new to aquaponics but for what its worth. I have two systems with 5 cement mixer tubs that are about 3'x2'x9" grow beds with a 30 and 50 gallon fish tank. The 30 gallon one over flowed but just a little. The 50 gallon one was better. Maybe someone can figure out the ratio. I use a 520 gpm pump, five valves to each grow bed, flood and drain with bell siphon. see my videos. Lloyd

[TCLynx]

I'm guessing you actually mean a 520 gallon per hour rather than gpm, 500 gpm would probably spray the gravel right out of those tubs.

 

When people talk about media bed to fish tank ratios, we are usually talking about total volume of the grow beds to the total volume of the fish tank.  Like 600 gallons of grow bed and 300 gallons of fish tank would be a 2:1 ratio.  With a 2:1 ratio you need some means to control water level fluctuations in the fish tank or you will be training the fish to breath air which is usually not good for them.  (Sump tank or indexing valve or constant flood becomes necessary if you go much beyond a 1:1 ratio.)

 

The usual estimate of how much water is needed to flood a grow bed once media is in it, would be 40% of the total volume.  The exact amount varies by type of media and amount of plant roots in the bed.  I usually look for 50%-60% of total grow bed volume when figuring how much sump tank would be needed for a system since you need some depth for pumps to operate and having a bit of extra depth for top ups or a automatic top up valve is handy.

[litano]

haha,, yes you are right on the pump. I looked the grow bins this morning, and the size is closer to 2'x3'.

TCLynx said:

I'm guessing you actually mean a 520 gallon per hour rather than gpm, 500 gpm would probably spray the gravel right out of those tubs.

 

When people talk about media bed to fish tank ratios, we are usually talking about total volume of the grow beds to the total volume of the fish tank.  Like 600 gallons of grow bed and 300 gallons of fish tank would be a 2:1 ratio.  With a 2:1 ratio you need some means to control water level fluctuations in the fish tank or you will be training the fish to breath air which is usually not good for them.  (Sump tank or indexing valve or constant flood becomes necessary if you go much beyond a 1:1 ratio.)

 

The usual estimate of how much water is needed to flood a grow bed once media is in it, would be 40% of the total volume.  The exact amount varies by type of media and amount of plant roots in the bed.  I usually look for 50%-60% of total grow bed volume when figuring how much sump tank would be needed for a system since you need some depth for pumps to operate and having a bit of extra depth for top ups or a automatic top up valve is handy.