Confusion over fish to growbed ratios...

[Sue McL]

Okay, I'm new to this and haven't built my first system yet (other than two 10 gallon tanks indoors) but can someone please help me understand the ratios?

 

If I have 50 fish in a hundred gallon tank, my growbed should be 100 to 200 gallons in volume, right?

 

But if I have the same 50 fish in a 250 tank, my growbed should be 250 to 500 gallons? Why? Same # of fish, same amount of poop, just the dilution has changed to make the fish happier....

 

My "dream system" will have 5 FT and 5 GB, orginally planned with 150 gallon FTs. Now I'm investigating using IBCs (if I can find them) but would almost double the amount of water available -- fish would remain the same. So do I have to increase my GBs?

 

I may just be over-analyzing this but that's all I can do until I have the funds to build my greenhouse setup.

 

Thanks in advance for your feedback.

Sue

[TCLynx]

Take a deep breath.  You may simply be confusing a few different ways of explaining things which is what is probably upsetting you.

The most important part is to make sure you have enough filtration for your fish load so yes, you want to base your amount of fish on the amount of filtration you are going to have (setting aside the whole issue of water level fluctuation and sump tanks for now.)

 

Many people will plan a system based on fish tank size (rather than thinking about the fish load ahead of time) and in those cases we generally try to recommend the media bed filtration be equal volume to fish tank (or twice as much if water level fluctuation is dealt with) so that the system will be able to support close to the max amount of fish for the amount of fish tank water.  So many people will say "I have a fish tank XX size, how many fish can I put in it" and this is why we usually encourage people to max out their filtration.

 

Up to a point you can get away with a bigger tank and less filtration provided you keep the fish stocking low enough so you won't be overloading your filtration.  Like having 50 cubic feet of grow bed to support your 50 fish growing out to a pound each be they in a 200 gallon fish tank or 600 gallon fish tank.  However, as I said this is to a point.  If you have a 10,000 gallon pool and you are expecting a 300 gallon grow bed to keep the water nice, you may struggle even if you only have 10 fish in there since other things can affect water quality, like the amount of flow or turn over of the water per hour or day and a big pool is kinda hard to filter through a small amount of grow bed each hour so the algae, debris and other things in the pool can cause issues if things are too out of balance.

 

Anyway, if you can resist the urge to add more fish cause the tank is big enough, then having the appropriate amount of filtration for the grown out fish size along with a larger fish tank can be ok.

 

As a side note, I like systems with twice as much media bed as fish tank volume because they are usually really stable as far as water quality goes once they are up and running.

 

And I would feel uncomfortable about growing out 50 fish in a 100 gallon tank no matter how much grow bed it has.

[Sue McL]

Thanks!

 

I'm planning on happy fish and healthy plants. So will give each pound of mature fish six gallons of FT water and have 2:1 GB:FT ratio. That's a "do-able" number.

 

Whew!!! I was really going around in circles...

 

Now to figure out how to heat in winter or if it's even worth trying... Will be back with more questions -- later!

[TCLynx]

I highly recommend choosing a fish that doesn't need warm water to survive (not sure that you would think of 55 F water as warm but that is just to keep tilapia surviving not doing much else.)  Choose a fish that can survive cold water and then all you need to do in the way of heating is keep your water, pipes and inlets to the grow beds from freezing and the system can get along at a low tick over winter and will probably still manage to grow a few cold weather crops slowly over winter.  I'm assuming greenhouse based on your state, OR.  Are you near the coast or inland or up in the Mountains I suppose that will make a big dif about how much heating would be required to keep things from freezing over.

[Sue McL]

I'm in the high desert of Southern Oregon, eight miles north of the Cali line, south of Crater Lake. The coast is four hours away. Elevation only 4700 feet.

 

We only get about a foot of snow at a time -- generally end up with five or more feet on the ground over the winter. Bad winters we can get 5 - 6 feet at a time. Average winter temp is 20 with at least a week or two sub-zero.

 

Only cold fish I know of or are available are Salmon, Trout or Arctic Char. Not sure if I have the knowledge to keep them alive. Guess I'll learn!

 

Was hoping for warm water fish and the ability to winter them over inside the house / garage / pump house but... We are also extremely limited on what OFW will allow to be raised.

 

Thanks again!

[TCLynx]

Well there are some warm water fish that over winter in lakes that freeze over so you are not limited to trout, though I understand they can be great fish to raise.

 

Catfish and bluegill are two warm water fish that come to mind that can survive cold water (I grew up in Michigan and the lakes freeze over there and the bluegill, sunfish and bullheads survive winters.)

So if you are doing a greenhouse you could probably shut down all but some aeration and minimal tank over winter and perhaps even overwinter some fish right in a large tank in the greenhouse and then start things back up as spring warms up again.  The system will be a bit slow starting up while things are still cold but that just means monitoring water quality while you slowly increase feed as the water temps warm up.

 

If you go with something like tilapia, you would definitely need to bring them in where the water temperature would stay at least above 55 F.