More, deeper media beds the more fish you will be able to support.
For gravel filled strawberry towers, I would probably not count them as much filtration since water trickling through gravel tends to channel so you don't get as much filtration out of such towers as you might out of a Zipgrow tower which is designed to be filtration.
Don't count NFT tubes as filtration, they require the water to be well filtered before you send the water to them.
To be able to support fish well, I would recommend about a cubic foot of media bed per fish (if you are only growing the fish out to 1 lb each.)
So if your media bed is only about 19 cubic feet of media and then your barrels if only using 2/3rs of a couple of them gives you maybe 23 cubic feet more media for a total of 42 cubic feet of media bed. So that would be up to 42 lb of fish. And then you get to maybe have another pound of fish per each 3 square feet of raft bed so add 8 which gives you a grand total of about 50 lb of fish/aquatic livestock MAX grown out size in this system. If you go with 6 gallons of fish tank per pound of fish, that means you need a total of 300 gallons of fish tank. I think of 300 gallons of fish tank as a good minimum size fish tank personally since it is big enough to be fairly temperature stable and it is the minimum size I personally like to grow out channel catfish which are my fish of choice. I would only stock about 15 channel catfish into such a system though since I tend to grow them out to at least 3 lb each. I won't stock more than 1 channel catfish per 10 gallons of fish tank for grow out anymore either.
If your system water is cool enough to keep trout happy, it is too cool for actually growing tilapia out to a decent size.
Let me repeat,
If your water is cool enough to grow trout well, you are going to be lucky to get 4-8 oz tilapia. Tilapia need their water to be over 70 F for them to be eating much at all and you don't get to grow out tilapia to eating size in only a few months if the water is not up in the Mid 80's which would likely kill trout in the same system.
Trout if conditions are good for trout, actually can grow out to edible size in a partial season (though probably only to around 1 lb.)
Tilapia from an advanced fingerling stage of All Male stock might be able to reach 12 oz in a partial season under their ideal conditions.
Unfortunately when I say a partial season I'm talking 9-10 months. Ideal conditions for Trout are definitely NOT ideal conditions for Tilapia. I have a feeling if you try to run a system that will keep both alive, neither one is going to thrive enough to be able to harvest in a single partial season. To get the water warm enough to be able to harvest tilapia in less than a year, will likely make it so you can't actually keep the trout alive year round either.
As far as being able to run through the colder months, trout will likely provide plenty of nutrients to keep the plants going. Remember Trout need high protein feed so you actually need more filtration and plants to keep up with trout through the cooler months provided your system is not getting so cold as to require dormancy all around.
It really doesn't take that many fish to grow a heck of a lot of plants. Keeping in mind that some plants use more nutrients than others. But in general, people put in too many fish and not enough grow beds or plants in most cases. If you find that you don't have enough nutrients available you can always just not plant as much during that season. Keep in mind that if it is cold and cloudy, you don't want an overabundance of nutrients in the water since it can actually make some greens more toxic.
When I said 1 cubic foot of media bed per fish that might get to a pound, that is the MAX recommended for a new system. Or if you prefer to think in liters, 25 fish per 500 liters of grow bed provided you are only growing the fish out to 500 grams each.
I have access to a 450 gallon Stainless Steel Dairy tank. I have heard that you don't want to use certain metal in your system. Is Stainless Steel a safe metal? I brew my own beer, Stainless is the safest metal to use for that.
I've heard of people using stainless steel dairy tanks for aquaponics. Seems overly $$ though for something that isn't going to be all that easy to plumb through since the existing plumbing is rarely of the appropriate size or position or fittings.
Thanks for all the help with my tanks. I went out and got 2 330g IBC tanks that will be my FT's. I have a few more questions if you don't mind.
1) With oxygen: I was wondering if the rules still apply for pressure needed if you go through the side of the FT, with a bulkhead fitting then attach your air stone with a backflow preventer vs. going up over the top of the FT down the full volume to the air stone. Also I read about oversaturation. I have some Stainless Steel brewing Carbonation stones. They are a .5 micron. If I used these, one in each tank, will that oversaturate the water with oxygen? Would it be feasible to put them on a timer to avoid oversaturation?
2) With my 2 FT's to keep the water level equalized, I was going to use 3" pipe at the bottom of the FT. Will I need to put something over the pipe to keep the fish from swimming back and forth? The fish will all be the same, maybe. Or should I use a bigger size so the fish can swim back and forth without getting to big for the pipe?
3) I am going to use different types of beds in my system. I am going to use media beds and float table. I am trying to use 1/2 barrels, cut short way, for trees and bushes. I am trying to wrap my head around how to set those up. Should they be on a continuous drip with drain holes in the bottom or flood and drain like traditional media beds? Would it depend on the plant going in the barrel and what they like for watering?
This is a diagram of what I want my system to look like, or something similar. Since drawing this up and going over it with my Dad, who is very good with plumbing and the way of water, I have changed a few things already. The sump tank will actually be a 275g IBC tank buried. The main pipe size will be 1", is that to big? the dump return line will be 1 1/4" for better flow. Any insight or suggestions will be appreciated. We got our tax return and I want to buy some more parts.
If this is your first system I would start with one IBC fish tank and enough GB to support it. You won't need a sump or filter unless/until you have more fish than the bed can support. Then you can add more GBs. Once you're operating you will realize more practical applications for growth as you learn. I started my system last year with 1 IBC using the top 12" flipped for the grow bed. Started with 5 Tilapia and now have 800+. I've had to concentrate on raising fish and not much attention to veggie but the plants don't require much attention. I've read all the blogs about people losing fish and I haven't lost a single fish to disease. I added a second tank just to be able to separate different size fish. It's been a learning curve for sure. This year I will be extending my GH and adding GBs trying alternate media.